hard to say, man. Gracie Barra has around 600 black belts according to Wikipedia, with 400 school world wide. Alliance has only 13 schools, but being world championship team for some many years in a row, I wouldn't be surprised if they have 100 black belts between those 13 schools. Checkmat is probably up there too. Plust there are dozens of small organizations with hundreds, if not thousands black belts. I live in St Louis, MO, and we have 13 Black Belts that I know of.
My guess would be 5000-7000?