It's 2 because there cannot be just one. There must be 2. Same as with magnetic poles and Siths. (and not same as with Highlanders.)

It's not infinite, because if you add more, you get a different space of numbers.

j and k are not complex numbers, in the same way that i is not a real number.

Changing the name of a number doesn't change the number. "Two" is not a different number from "2".