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.
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.)