The question was about humans. But yeah, it is limited to two. As in, there are never more than two. Some creatures can produce asexually as you say, some can switch between the two sexes if there aren't enough males/females around, but none require an egg, a sperm cell, and something else to reproduce.

But again, none of that matters really, because the question was about humans. There are two sexes in humans and all mammals. I don't understand how we've become convinced that this question is suddenly complex.