Corrected entry: In the chapter 'The Rogue Bludger', Harry breaks his arm in the quidditch match against Slytherin and Madam Pomfrey said "This boy needs rest, he's got thirty-three bones to regrow!" There are only 30 bones in an arm.
Greg Dwyer
21st Jan 2018
Correction: There are 30 bones in the arm and hand. Add in the clavicle and scapula for two more. There are also likely to be sesamoid bones - bones embedded in tendons or muscles that develop in response to stress or strain. Harry has certainly been through plenty of both. She could also be referring to the epiphyseal plate, AKA the growth plate.
Greg Dwyer