Here is a partial list by population https://en.wikipedia.org/wiki/List_of_primates_by_population
Maybe 1 to 2 million? You can add up everything on the list (exclude humans and other non-monkey primates like Gorillas/Apes) for a better estimate. The list is not complete since nobody really knows the number of all the different monkey species, but I would think most of the ones not studied well have small numbers.
