It's difficult to know how many subscribers are paying for the game. A couple months ago, someone did a Lodestone census (compiling data from it) that suggested around 700k people were actively playing (but there may be more subscriptions current than that). In a vacuum, that number's not very useful, of course. WoW stopped providing subscription numbers as of this year, and we don't know how many of those subscriptions were active players even back when they provided them.

We can guess that the game's successful, at least, as it's the only subscription-based game in recent memory that hasn't moved to a free-to-play model that I can think of, and it's been out for 2.5 years now.

The "5 million" accounts number includes existing accounts that aren't actively paying for the game, I'm pretty sure. My guess would be that, at most, the game has between 750,000 and 1 million subscribers right now, which is fairly impressive for a subscription game in the era of free to play stuff.