For Legacy... I think it's around ~20k, because there are a number of JP players with Legacy as well. I estimate were around 40000 active XIV players towards the end (I count active as logging in once a month).... though in the last couple months leading up to the finale, there were a lot of new players/unfamiliar names. Difficult to say, really.
I might be an onion thief, but I'm still a thief.™