Stupid Question:
Reincarnation is basically the recycling of souls. If the total population of stuff-with-souls grows, where do the 'new' souls come from?
Depends on the specific metaphysics. Sometimes there's mechanism for new souls to show up one way or another (more created, old ones fragmenting, etc.), sometimes there's not and there's always been a (massive) number waiting for something to get stuck in, sometimes there's not and some things just end up soulless p-zombie equivalents. Sometimes the question isn't really addressed and maybe you get whacked with something for asking questions the person you're asking don't want to answer.
So yeah. It depends on who you're asking and the metaphysical assumptions they're operating under. Not really a single catch-all explanation.