Question on Reincarnation.

I have an interest in reincarnation but I have a question that troubles me. When humans first populated the Earth and the population was sparse and we were spirits. Where did we reside and how long before we were re-born? I have this idea that there must be Billions of spirits waiting to be re-born which means at some point the Earth will be vastly overpopulated. Obviously that cannot continue so how is this to be avoided? . The other thing I have been thinking about is when the Universe dies and is reborn where will we be? I hope you can understand what I am getting at as reading it back it doesn't look very legible.

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