World of Warcraft is the biggest MMORPG, at one point topping 12 million subscribers. There down a bit from that, but it's common when one expansion has run its course and another is due out in a few months. But Blizzard shouldn't worry too much about the drop in subscribers, with the Legion expansion only three months away and this little game called Overwatch coming out next week.
How big is Overwatch? 9.7 Million players jumped in on the recent beta from over 190 countries. 4.9 Billion minutes played over the short window that the beta was up. That is the equivalent over 9,000 years. As for the most popular characters: Soldier 76 for offense, Widowmaker for defense, Reinhardt for tank and Mercy for support. I would've expected to see Tracer, Reaper or Winston in that group.
Even though you needed to pre-purchase the game to get into the beta, I have heard from multiple sources that each person got more than one key. I ended up with three from my purchase from Amazon. So I don't think you can equate the number of players to the number of sales. But the interest for the game seems to be there. We'll get a better idea of the popularity when the game goes live next week.