Dragon Slayers, God Slayers, and now Devil Slayers… How many types of Slayer will there b

Dragon Slayers, God Slayers, and now Devil Slayers… How many types of Slayer will there be?! Silver kind of looks like Gray. Probably his relative, different world self, or future self. I mean he has the same eyes, color of hair, type of jewelry, type of magic like Gray. And the name as in color is very similar to each other. Silver… Gray… (Silver IS shinier though LOL) IDK. What do you guys think?