I can see other players (friends) on the leaderboard with levels from 27 to 34 and ranking from 40,000 to 87,000. So there are at least 87,000 registered player names. It appears that the players, level 34 and below that I can see, have not played for quite some time as their "games played" number has not changed. If that is the case then there are less than 40,000 active players. Which is still a staggering number.
Sometimes I think there must only be a dozen active players when I am paired with the same ones again and again.