Board Thread:General Discussion/@comment-36186343-20200309075755/@comment-1429792-20200321173707

most of that has never been touched upon in any detail. The only thing that I can remember is that Mayuri killed 25-30 thousand people in Rukongai during the final arc's early stages to restore the balance after the Wandenreich attacked Hueco Mundo.

So you would have to make a best guess at all the rest. Id suggest look up the real world stats on natural death rate and assume (making up a number here) 90% of them would be sent to Soul Society and the rest turn to Hollow.