That sounds too high, just from a space and traffic management viewpoint. I found figures of 1300, 1700, and 1800 on the net, although none of these seemed super reliable. Based on maps I have seen of the White House, a lot of these people must work at the EOB.
The entire ‘campus,’including OEOB is considered “the White House” when speaking of working at or having a meeting at the WH. It’s not just the WH itself.