I think if reincarnation was/is real it’d probably work regardless of time and creatures would be locked into their own categories. So like, humans could only reincarnate as other humans, for example.
When I say it’d work regardless of time, I mean like, if you died you could reincarnate as someone in the future or the past.
Kinda a silly discussion topic, but I thought it might be kinda interesting.
Since the population of the universe is not stable, either there’s some leeway to make more or less souls (making it not a true reincarnation system) or time is not a consideration for reincarnation.
We then have to consider the number of souls in the system. Enough for everyone? Two? Just one soul?
Finally the mechanism for what you are reincarnated into next is either completely random, or it is guided by you or some other entity. I imagine if it’s guided by you, you get what you want, and if it’s guided by Them, They get what They want.
There is a 3rd explanation to the population count: we don’t have the entire set of souls here. Having a universe of souls where planets ebb and flow in population would cover that problem (although without something naturally reducing populations at some point, one would expect to run into the same problem as before as successful populations become more successful and increase in scale.)