That's impossible to estimate, because publishers seldom give figures, and if they do, it is with a 6 to 12 months delay. Nice guys, hey?
I prefer to say that I don't estimate to more than 2 hundreds the active players of NCP circulating in the forum, with perhaps 50 actively posting at most.
If you can contact this Polish community, then this would be greatly appreciated. We can offer to the webmaster a copy of NCP for his own evaluation also.