In Buddhism, there are many other realms of existence like the hell realms, realms of the gods, the pure lands, the animal realm which physically exists in the same one as humans but is a distinct experience, etc. Insects also count in this.

Although similarly in the physical human and animal realm, aliens in an infinitely far away galaxy would fall under the same rules. So you've got a lot of pools to pull from that are all impermanent, but not bound by human population numbers.

Edit: there's also two distinct schools of thought as to whether rebirth is instantaneous after you die or if there's a sort of lag that may be intentionally affected by those who are sufficiently enlightened. So there could be a period of people sort of waiting in line, though its not actually that simple.