7 is the most powerful magical number.  Here is a list of things that have to do with the number 7.

7 books

7 years at Hogwarts

7 dedications to book 7

7 Horcruxes

7 members on a Quidditch team

7 is Harry’s number on the Gryffindor Quidditch team

7 Potters as of the epilogue of Deathly Hallows

7 Weasley children of Molly and Arthur

7th month Harry’s birthday is in

7th month J.K. Rowlings birthday is in

7 floors in Hogwarts

7th floor Gryffindor common room is on

7 is the age young witches and wizards should start to show magical power by

7th chapter of Standard Book of Spells that Alohamora is in

7 muggles---the number of muggles Harry and Ron were seen by when they drove the flying car

7 hidden passegways shown out of Hogwarts on the Marauder's Map