Well, I've read that it's actually gone a bit down from it's peak (some sketchy estimates put the active player base at 12 million, down from a 20 million peak), as games such as Hearthstone and fatigue eroded the playerbase, but Dominaria did bring some people back and it's on the upswing again - and with the new Hearthstone-like client they might get back to 20 million again.