Multiples, amigurumi rules...?

i have been making amigurumi toys for quite a while now but havent been able to understand why the basic rule to begin is to start by using multiples of 6. yet i find in other crochet patterns many start in multiples of 7 or more. what is the purpose of the multiple number? do the change in multiples create different shapes? i hav found many multiples end up creating the same shape. i hope im making sense lol