Why are there 7 chaos emeralds?

Answer 1

In many cultures including Japanese, Seven is a number that means good luck, that's why seven chaos emeralds are essential for the Sonic gaming experience, or quite frankly on another note, Seven Chaos Emeralds just sound cool, imagine any other number, "We must battle to find the eight chaos emeralds" it doesn't sound right.

Answer 2

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.