I say it's around 8millionish. Lots of people came back to a different WOW. People from WOTLK or Cata that never came back since. Server issues and lack of content has made the game undesirable. Everything is Garrisons now, or is a huge focus in this expansion. As much casual content that exists, if it doesn't help your character get better at fighting then it's not content. It's a waste of time.