Seven
From LeakyPedia
The Reoccurence of the number Seven in the Harry Potter books
The number seven is seen multiple times in the Harry Potter Series including but not limited to: Seven Books, Seven Horcruxes, Seven Weasley's, .... (more)
In Arithmancy students at Hogwarts School of Witchcraft and Wizardry learn about the magical properties of numbers, including that of the number seven.