Why are there 7 chaos emeralds

7 is probably a magic number. It's a low prime number. It's not 5, that may stand for a pentagram, that may be referred to as evil. Not 3, that may stand for a triangle, that can also be evil, as the "evil eye". Or 9 that may be inverted to be 6 as the 666 for the devil. May be a reason.

Also, it just might be an emerald for every color in the rainbow. Where there are 7 from.

Does it need another reason? It's not something real.