So I actually did some research through the bureau of labor statistics, and as of end of year 2016, the answer to this question is: less than 13,400 people in the USA, not include prop players.
How I found this:
Go to data.bls.gov
nerd out.
end up with:
We assume all professional poker players are self-employed, so only look at the 13,400 figure. This figure also includes other self-employed people:
* Acrobat
* Aerialist
* Amusement Park Entertainer
* Bareback Rider
* Baton Twirler
* Clown
* Comedian
* Continuity Director
* Fortune Teller
* Impersonator
* Juggler
* Magician
* Marionette Performer
* Palmist
* Professional Poker Player
* Prompter
* Puppeteer
* Rodeo Performer
* Story Teller
* Stunt Performer
* Tumbler
* Ventriloquist
* Wire Walker
But many of these people may be employed by a casino or comedy lounge or something, so eh.
Anyway, less than 5,000 in the USA is my best guess. Also, if you are a pro poker player, the government thinks your job is similar to that of a fortune teller, impersonator, ventriloquist or wire walker. Go think about that for a few minutes.