Seven

According to Tom Marvolo Riddle, seven is the most powerful magic number. It is the number of years in a sorcerer school like Hogwarts, the number of pieces Voldemort split his soul into (doing six Horcruxes).

The fans expected the seventh book to be released the 07/07/2007 (works in american and in european date format!) but sadly it was only released on 21st July 2007.