The number twelve appears quite often in the Harry Potter books. These are the mentions of the number twelve that I have found so far. I did include twelfth, dozen and half-dozen, but that is as far as I have stretched this list.
 

"Twelve times he clicked the Put-Outer, until the only lights left on the whole street were two tiny pinpricks in the distance." (SS1)

"It was a very odd watch. It had twelve hands but no numbers; instead, little planets were moving  around the edge." (SS1)

"He clicked it once, and twelve balls of light sped back to their street lamps so that Privet Drive glowed suddenly orange and he could make out a tabby cat slinking around the corner at the other end of the street." (SS1 )

"I must have passed a dozen feasts and parties on my way here."(McGonagall) (SS1) 

"On Friday, no less than twelve letters arrived for Harry." (SS3)

"Twenty-four letters to Harry found their way into the house, rolled up and hidden inside each of the two dozen eggs that their very confused milkman had handed Aunt Petunia through the living room window." (SS3)

"Around half past twelve there was a great clattering outside in the corridor and a smiling, dimpled woman slid back their door and said, 'Anything off the cart, dears?'" (SS6)

"Considered by many to be the greatest wizard of modern times, Dumbledore is particularly famous for his defeat of the dark wizard, Grindelwald in 1945, for the discovery of the twelve uses of dragon's blood, and his work on alchemy with his partner, Nicolas Flamel." (SS6)

"'Come back, boy!' she shouted, but Neville was rising straight up like a cork shot out of a bottle -- twelve feet -- twenty feet." (Madame Hooch) (SS9)

"They hadn't gone more than a dozen paces when a doorknob rattled and something came shooting out of a classroom in front of them." (SS9)

"It (the Troll) was a horrible sight. Twelve feet tall, its skin was a dull, granite gray, its lumpy body like a boulder with it small bald head perched on top like a coconut." (SS10)

"There are some things you can't share without ending up liking each other, and knocking out a twelve-foot mountain troll is one of them." (SS10)

"Festoons of holly and mistletoe hung all around the walls, and no less than twelve towering Christmas trees stood around the room, some sparkling with tiny icicles, some glittering with hundreds of candles." (SS12)

'"You're worth twelve of Malfoy,' Harry said." (SS13)

"Neville went bright red but turned in his seat to face Malfoy. 'I'm worth twelve of you, Malfoy,' he stammered." (SS13)

"It was hard to relax with Hermione next to you reciting the twelve uses of dragon's blood or practicing wand movements." (SS14)

"All the same, he'd have gambled twelve Sorcerer's Stones that Snape had just left the room, and from what Harry had just heard, Snape would be walking with a new spring in his step -- Quirrel seemed to have given in at last." (SS15)

"Flitwick told me in secret that I got a hundred and twelve percent on his
exam." (Hermione) (SS16)

"In fourth place, Gryffindor, with three hundred and twelve points;" (Dumbledore) (SS17)

"The Dursleys hadn't even remembered that today happened to be Harry's twelfth birthday."(CS1) 

"Dear Mr. Potter, We have received intelligence that a Hover Charm was used at your place of residence this evening at twelve minutes past nine." (MOM) (CS2)

"'I've asked you to stay about twelve times, and then Dad came home and said you'd got an official warning for using magic in front of Muggles---'" (Ron)(CS3)

"'His exam results came in the day before you did: twelve O.W.L.s and he hardly gloated at all.'" (Fred) (CS4)

"'Bill got twelve, too. If we're not careful, we'll have another Head Boy in the family.'" (George) (CS4)

"After a quick half a dozen bacon sandwiches each, they pulled on their coats and Mrs. Weasley took a flowerpot off the kitchen mantelpiece and peered inside." (CS4)

"'You have told me this at least a dozen times already,' said Mr. Malfoy, with a quelling look at his son." (CS4)

"Dozens of heavy spellbooks came thundering down on all their heads;" (CS4)

"Ron looked wildly around. A dozen curious people were still watching them." (CS5)

"'But when I was twelve, I was just as much of a nobody as you are now,' said Gilderoy Lockhart." (CS6)

"'And a few of you need to read Wanderings with Werewolves more carefully - I clearly state in chapter twelve that my ideal birthday gift would be harmony between all magic and non-magic peoples - though I wouldn't say no to a large bottle of Ogdeds Old Firewhisky!'"(Lockhart) (CS6)

"In the small vegetable patch behind Hagrid's house were a dozen of the largest pumpkins Harry had ever seen.: (CS7)

"Through the dungeon wall burst a dozen ghost horses, each ridden by a headless horseman." (CS8) 

"Not only were there a dozen frost-covered Christmas trees and thick streamers of holly
and mistletoe crisscrossing the ceiling, but enchanted snow was falling, warm and dry, from the ceiling." (CS12)

"Lockhart clapped his hands and through the doors to the entrance hall marched a dozen surly-looking dwarfs." (CS13)

"'This is an Order of Suspension - you'll find all twelve signatures on it.'"(Lucius Malfoy) (CS14)

"'That remains to be seen,' said Mr. Malfoy with a nasty smile. 'But as all twelve of us have voted -'" (CS14)

"'Harry... She bought you twelve years of borrowed time ... but Lord Voldemort got you in the end, as you knew he must . . . .'" (Tom Riddle) (CS17)

"'Excellent nosh, Petunia. It's normally just a fry-up for me of an evening, with twelve dogs to look after....'" (Aunt Marge) (PA2)

"Harry was trying to remember page twelve of his book: 'A Charm to Cure Reluctant Reversers'." (PA2)

"There were no seats; instead, half a dozen brass bedsteads stood beside the curtained windows." (PA3) 

"... the magical community lives in fear of a massacre like that of twelve years ago, when Black murdered thirteen people with a single curse."  (Daily Prophet) (PA3)

"'Anyway, they cornered Black in the middle of a street full of Muggles an' Black took out 'is wand and 'e blasted 'alf the street apart, an' a wizard got it, and' so did a dozen Muggles what got in the way.'" (Stan Shunpike) (PA3)

"The door of number twelve was ajar and Percy was shouting." (PA4)

"Black lost everything the night Harry stopped You-Know-Who, and he's had twelve years alone in Azkaban to brood on that...."(Arthur Weasley) (PA4)

"Trotting toward them were a dozen of the most bizarre creatures Harry had ever seen." (PA6)

"Without warning, twelve-foot wings flapped open on either side of Harry, he just had time to seize the hippogriff around the neck before he was soaring upward." (PA6)

"Harry wasn't surprised; he could see that it was crammed with at least a dozen large and
heavy books." (PA7)

"Slowly, she reached down for a bag at her feet, turned it upside down, and tipped a dozen bits of splintered wood and twig onto the bed, the only remains of Harry's faithful, finally beaten broomstick." (PA9)

"Did he realize he was facing twelve years in Azkaban, twelve years that would make him unrecognizable?"(Harry) (PA11)

"When they went upstairs to the crowded common room, it was to find Fred and George had set off half a dozen Dungbombs in a fit of end-of-term high spirits." (PA11)

"At lunchtime, they went down to the Great Hall, to find that the House tables had been moved against the walls again, and that a single table, set for twelve, stood in the middle of the room." (PA11)

" ... and the Great Hall was filled with its usual twelve Christmas trees, glittering with golden stars. "(PA11)

 "Mrs. Weasley had sent him a scarlet sweater with the Gryffindor lion knitted on the front, also a dozen home-baked mince pies, some Christmas cake, and a box of nut brittle." (PA11)

 "'No, Potter, you can't have it back yet,' Professor McGonagall told him the twelfth time this happened, before he'd even opened his mouth."(PA12)

" ...  like a skeleton, with loads of filthy hair ... holding this great long knife, must've been twelve inches..." (Ron Weasley) (PA14) 

"Black had proved twelve years ago that he didn't mind murdering innocent people, and this time he had been facing five unarmed boys, four of whom were asleep. (Harry) (PA14)

"'THE DARK LORD LIES ALONE AND FRIENDLESS, ABANDONED BY HIS
FOLLOWERS. HIS SERVANT HAS BEEN CHAINED THESE TWELVE YEARS. TONIGHT,'" (Trelawney) (PA16)

"'Peter Pettigrew's dead!' said Harry. 'He killed him twelve years ago!'" (PA18)

"'I've waited twelve years, I'm not going to wait much longer.'" (Sirius) (PA18)

"'Twelve years, in fact,' said Lupin. 'Didn't you ever wonder why he was living so long?'" (PA19)

"'I knew he'd come after me! I knew he'd be back for me! I've been waiting for this for twelve years!'" (Peter Pettigrew) (PA19)

"'You haven't been hiding from me for twelve years,' said Black." (PA19)

"'I must admit, Peter, I have difficulty in understanding why an innocent man would want to spend twelve years as a rat,' said Lupin evenly." (PA19)

"Something was slithering towards him along the dark corridor floor, and as it drew nearer to the sliver of firelight, he realized with a thrill of terror that it was a gigantic snake, at least twelve feet long." (Frank Bryce) (GF1)

"By twelve o'clock the next day, Harry's school trunk was packed with his school things and all his most prized possessions." (GF4) 

"'The Bulgarians are insisting we add another twelve seats to the Top Box.'" (Crouch Sr.) (GF7)

"'Ali thinks there's a niche in the market for a family vehicle', said Mr. Crouch. 'I remember my grandfather had an Axminster that could seat twelve - but that was before carpets were banned, of course.'"(GF7)

"'As if it wasn't specifically stated in paragraph twelve of the Guidelines for the Treatment of Non-Wizard Part-Humans --'"(Arthur Weasley)  (GF10)

"'They want compensation for their ruined property. Mundungus Fletcher's put in a claim for a twelve-bedroomed tent with en-suite Jacuzzi, but I've got his number.'" (Percy) (GF10)

"lost," "hospital," "prison," and, in the position where the number twelve would be on a normal clock, "mortal peril."(GF10)

"Apart from The Standard Book of Spells, Grade 4, by Miranda Goshawk, he had a handful of new quills, a dozen rolls of parchment,"(GF10)

"'Yeah. . . cool. . .' said Harry, scribbling it down, 'because... Venus is in the twelfth house.'"(GF14)

"they saw a gigantic, powderblue, horse-drawn carriage, the size of a large house, soaring toward them, pulled through the air by a dozen winged horses, all palominos, and each the size of an elephant."(GF15)

"Harry, whose attention had been focused completely upon Madame Maxime, now noticed that about a dozen boys and girls,"(GF15)

"Next thing he knew, he was being wrenched inside the common room by about a dozen pairs of hands,"(GF17)

"'Containing a single hair from the tail of a particularly fine male unicorn. . . must have been seventeen hands; nearly gored me with his horn after I plucked his tail. Twelve and a quarter inches. . . ash.'"(Mr. Ollivander) (GF18)

"He would be speaking to him in just over twelve hours, for tonight was the night they were meeting at the common room fire - assuming nothing went wrong, as everything else had done lately..."(Harry) (GF19)

"'you need about half a dozen wizards at a time to overcome a dragon -'")Sirius) (GF19)

"Pigwidgeon plummeted twelve feet before managing to pull himself back up again;"(GF21) 

"Everlasting icicles had been attached to the banisters of the marble staircase; the usual twelve Christmas trees in the Great Hall were bedecked with everything from luminous
holly berries to real, hooting, golden owls,"(GF22) 

"'Why do they have to move in packs?'" Harry asked Ron as a dozen or so girls walked past them, sniggering and staring at Harry. "(GF22)

"The House tables had vanished; instead, there were about a hundred smaller, lantern-lit ones, each seating about a dozen people.") (GF23)

"Half a dozen mermen were pulling him away from Hermione, shaking their green-haired heads, and laughing." (GF26)

"they had sneaked a dozen chicken legs, a loaf of bread, and a flask of pumpkin juice from the lunch table."(GF27)

"Half a dozen house-elves came hurrying forward, looking disgusted."(GF28)

"'We has your extra food!' squeaked an elf at Harry's elbow, and he shoved a large ham, a dozen cakes, and some fruit into Harry's arms."(GF28)

"because she might want to up the number of students she's bringing, now Karkaroff's made it a round dozen ... '"(Crouch Sr.) (GF28)

"'... and when you've done that, Weatherby, send an owl to Dumbledore confirming the number of Durmstrang students who will be attending the tournament, Karkaroff has just sent word there will be twelve. . . .'"(Crouch Sr.) (GF28)

"'Yes, my son has recently gained twelve O.W.L.S, most satisfactory, yes, thank you, yes, very proud indeed.'" (Crouch Sr.) (GF28)

"Twelve feet tall, its face hidden by its hood, its rotting, scabbed hands outstretched, it advanced, sensing its way blindly toward him."(GF31) 

"It worked - the Disarming Spell made the spider drop him, but that meant that Harry fell twelve feet onto his already injured leg, which crumpled beneath him."(GF31) 

"From the interview transcript with JKR February 3, 2000: 
Q. Do you have a favorite passage from one of your books?
A. 'Hard to choose. I like chapter twelve of Sorcerer's Stone (The Mirror of Erised), and I am proud of the ending of Goblet of Fire.'"