That doesn't really address the question. Even if 5 billion people currently alive were previously some or other now deceased person, that means two billion were some sort of animal in the last life and four billion people now alive were some sort of animal in the last but one life and so on and if we add in all the other animals currently alive is there enough previous souls to go round ? This thinking is not consistent with what we know about the branching and diversification of life, if you go back far enough in time we get back to microbial life only and before that no life, so where do all these souls come from, and by what rationale do they hop from one deceased individual into another and what keeps them attached to one individual during a life ? Does the fact that I don't have any sort of noetic experience during yogic meditation suggest that I am one of the two billion being a human for the first time ?
I agree that you will not be able to match the soul count and come out with some sort of an equation. That is why I said that this is not Physics. Stop thinking in terms of such equivalences.
There is no complete theory of reincarnation which gives detailed account of such matters. Nor do I think it will ever be done.
Such scornful questions can be asked about Strings, 11 dimensions, Parallel Universes, Dark Energy and so on, even though they are part of an exact science like Physics. That is what makes the universe so complex. Simplistic ideas don't work.
However, if Tucker and his team have successfully investigated 2500 cases and found suitable matches...that is evidence of reincarnation, period! How the maths works out, I have no idea.