there is no way to say. blizzard wont release that info anymore, which is telling that it is obviously bad.
i been here since release so i met a crap ton of people over the years and since they added real id somewhere back in 2010 i have had somewhere close to 400±ish friends on there. right now today left online is probably 7 or 8 of them and 2 are just always on classic.
so if i just take a shot in the dark at it and if that is any indication to the overall big picture, i would say that wow across all wow platforms got somewhere in the ballpark of 400k to 1mil players. but then, it dont even feel like it has that much.