This post shows a distinct lack of knowledge about the subject.
Whether or not reincarnation is real, the idea is that people get reincarnated as people and any sort of living creature that exists. So, there's no need to be enough humans at any given time for the idea to hold, as long as there are enough living things. What's more it's been accepted theory for many centuries that only a very small fraction ones incarnations are as humans, most of the time it's as things like ants and spiders.
Or, that's the theory anyways, reincarnation is really more of a framework than a thing. It's not intended to be the driving force for ones life, just a framework for understanding how to live life in the context of the greater picture. You don't burn down the world because previous generations didn't and you have an obligation to future generations as well.