To my knowledge, SE has never released official sub numbers. The estimate I found quoting 730k was done by players here, which was over a year ago, before Heavensward released.
Heavensward could have easily doubled or triple that at some point. Myself and many others didn't really enjoy the base game and came back for the expansion. With the amount of people who play both games, I predict WoW and FF14 have an inverse relationship for subs... And WoW hasn't been too great lately.
The sub graph for for WoD here shows a dip of 5million in 9 months. Add another 9 months of very little content for the same expansion and we arrive to today.
If Blizzard were to post their subscription numbers today, they would have a nice buffer from tokens (I still have 6 months on my account, but have not logged in since Januaryish... Thanks to tokens). If they took that away today, or never introduced it, it wouldn't be wild to guess that their subs have dipped under 4million - maybe lower. Even with the tokens, it's possible that happened anyway.
They are playing a lot of cards in Legion, and there is a reason why. There is a reason they gave us tokens.
If Legion sucks and SE keeps making good changes and releases another expansion, why wouldn't they pass WoW? The old timers like me are starting to drift. I'll probably play the expansion, and they will probably get a boost.. but it's really getting to the point of no return and FF14 is just getting started.