Hearthstone has been out in full release for just six months, but according to Blizzard, the free-to-play card game has already amassed 20 million players.

The official Hearthstone twitter account tweeted out the statistic today in the above graphic, writing, “Thank you all for joining us in Hearthstone! We’re going to need a bigger tavern!”

That puts Blizzarrd’s experiment with a new gaming model well above the current number of World of Warcraft subscribers (which currently hovers around 6.8 million). However, Blizzard doesn’t specify how frequently those Hearthstone players are active, and as a free game, there’s no barrier to entry for anyone who wants to try out a few rounds.

Leading the current surge in interest was likely the single-player Curse of Naxxramas expansion that started rolling out in July, and the game keeps players coming back regularly by knocking everyone down in the leaderboards at the start of each month (but as long as you get to rank 20, you earn a new, colorful card back).

We’ll be sure to learn more about what Blizzard has in store for the game during November’s annual Blizzcon event, but we already know that the next expansion will focus on adding a large number of cards to players’ decks rather than a Naxxramas-like experience. And Hearthstone is due out on mobile devices some time this year.

But what do you want to see from Blizzard for its competitive card game in the coming months? Let us know in the comments below.