if i am not wrong, buddhism talks about 6 paths (animals,
ghosts, gods, humans, blah blah blah), can become humans
in their next life. so, you cannot track the number of souls
because it includes animals and others. any buddhists out
there, correct me if i am wrong.
besides the paths, there are also other world systems in
buddhist cosmology which he is ignorant about. besides it is
impossible to know how many beings there are in samsara even if you
attribute a number to it, so it can be said to be uncountable.