I am not sure you will get some numbers here. I have never seen any such statistics. You may be able to extrapolate based on IHRSA data of number of health clubs http://www.ihrsa.org/about-the-industry/ but that does not count the studios and those who freelance.
You might want to take a look at some of the data from the bureau of labor and statistics.
If this article does not fully answer the question you could look further on this site. There is quite a lot of data to be had here: