Anyone want to take a stab at this? I'd count anyone with a GJ guitar who plays it more than once a week :-)
My guess is less than 1000, maybe less than 500. If every major city in the country has 10 players and there are maybe 25 major cities where I know of a GJ presence and then multiply by two to take care of everyone we missed outside the major cities, I get 800.
Here in DC, there are maybe 10-15 players and I think of DC is as being one of the more active areas. Baltimore probably has another 10 thanks to the efforts of Michael J Harris and Sami Arefin who host a weekly jam session.
On the other hand, I don't get out as much as I should, could be way off. Maybe some of the touring players here can chime in. I wonder about the big cities like NYC and Chicago. And places I think of as very active like Seattle and Portland. San Francisco maybe. Maybe these places have dozens and dozens, I dunno.
I'm thinking it is less rather than more. Surprise me.