I would love to get straight answers to these, and many other questions too. Unfortunately, the Gwent team, or CDPR itself, is quite evasive with these questions - whenever asked how Gwent is doing they always reply 'its doing well' or something along those lines.
You really need to do a lot of investigation and speculation to get to some numbers:
-I dont use Gwent reddit, but if it works like i think, you can see the number of registered users, which currently is 117k players. Obviously this isnt all the players, probably not even half, but gives a ballpark idea of what that number could be.
-The leaderboards also give some numbers, i dont pay much attention but in pro rank each month i think the number is in the thousands
-Recently, the creator 'PlainTalkJon' released a video with plenty of his polls, one of them asked what rank you are. The answers (with around 2.5k votes) were around 1/3 in pro rank, 1/3 in ranks 1-5, the rest below. HOWEVER, these numbers are probably not representative, as Jon mentioned, the players engaged in content are more likely to be playing in high ranks since they're more commited to the game.
(EDIT) - forgot to mention Twitch, it also gives some idea on the numbers of Gwent players. Compared to other games, they are quite underwhelming values, any trendy indie games that pops up quickly gets better results