Every so often someone asks this here, and none of us seem to know. But there was a thread about it a few months ago with a lot of discussion; have you tried the search function?

For a real solid answer I think you'd need to go to a chinese gov't source. If you don't speak chinese get a chinese person to help you find it through some kind of "city of Beijing" website or something. Certainly the city knows because they have all of us registered (mostly).

I read "several hundred thousand" someplace, and don't remember where, but that seems high.