The Problem With Reincarnation

There are about 86 billion neurons in the human brain, if I remember right. Let's say every single human that has ever been born or will ever be born has the exact same number of neurons. How many different life spans would we have to cover for 86 billion things to hit that exact same one permutation again? And then you get into like a whole problem of free will, which is not where this poem is supposed to go.