If we take Magha Puja day - there were 1250 (gathered in one place). During the First Council after Mahaparinibbana - 500 arahants (gathered in one place). So I think overall quantity is not much more than these numbers. Possibly 3000-4000 as maximum (during all his life).
In some sutta Buddha says that there are not one hundred, not two hundred, but more. He doesn't say there "thousand".