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We could all agree that 9 million active players would be physically hard to count even by a team of persistent, dedicated group of people trying to prove either way. HOWEVER I for one don't believe 9 million is even close and that the numbers can be counted. Let me explain why :
A few years back I decided to do my own style of research on player counts for other mmos such as Everquest 2, Vanguard, and Dungeons and Dragons online. Sure they are much smaller populated mmos. Sure I did not get a 100% accurate count, but I'm sure I was within + / - 10%.
Using the Social Panel > Who I had broken the list down to Level and Class as their would be a cut off of 49 players to do my count. Understanding that on a given time you would not have all active players on at the same time and this made it harder to get a full count. So I spent several days conducting my research. I would create a level one character on each faction for each server, and at different times and days. Prime time, 2 AM, mornings, along with other parameters that I can't remember. I was obsessed with this, but It was many years ago.
In my findings, I found that the numbers were NOT EVEN CLOSE to what people believed or what Developers were giving us. I had paperwork around my house as my proof for a long time, but as time went on it got lost and I forgot my actual numbers. I can say for fact that I was amazed at how low they really were. Such as the simplest count (Vanguard) was only like 280 as to 5K like people were lead to believe. Everquest 2 was way lower also.
Now With this, and knowing the number of servers in WoW and knowing that many realms are very low population along with doing a count on at least one populated server in the game, I could safely say that they only have about 1 million. Sure this is a GUESS. But the numbers can be achieved if a team working hard did a count.