The latest figures we have say they're current at eight million, but of course every bit of thinking we do on this issue is clouded by the expansion next week. Personally, I think the expansion will live up to the hype-- when hasn't Blizzard ever done that? Relmstein agrees with me, and says it'll bring a lot of players back. I'm thinking that means even players who don't expect to come back. Once the items start getting out there, and the new instances carve out their place in the culture, I think the Burning Crusade will bring this game back for a lot of people. Heck one indicator is the new PVP system-- if all the new content can revive the PVE game at least as much, it's a good thing Blizz is unwrapping all those new servers.
But of course the drawback to an expansion is this: you don't earn a new audience with addon content. I'd hazard a guess that the expansion will sell five to six million copies after all is said and done, but almost every one of those copies will go to someone who's already got an identity in Azeroth: I'd doubt anyone is sitting on the sidelines, just waiting for a Blood Elf or Draenei to come along and pick up the game. Of course, that doesn't mean Blizzard won't make money-- they'll rake it in, and hopefully spend it on even more servers and content. But I'm thinking that while Burning Crusade will bring a renaissance to Azeroth, total subscribers will top out at 10 million. And then, we can talk about another issue: will WoW's base peak in 2007?