It's difficult to get all of the actual sales numbers from throughout the years.
12 million WoW subscribers (a couple of years ago) does equal at least 12 million copes of WoW sold, and that is most likely just retail and not digital distribution. WoW has not always been available digitally. 1 subscriber equals 1 copy of WoW, that is pretty easy to figure out. The WoW expansions require a copy of WoW to have been bought. Also, even if WoW numbers are dipping to around 10 million, it's quite possible since the game is 8 years old that there have been more than 12 million copies of WoW (not the expansions) sold over time, probably something closer to 14-18 million with digital distribution (without it is probably around the 12 million mark). You have to remember that not every person that bought the original WoW or any of the expansions stuck with the game to create the different 6/8/10/12 and now back down to 10ish million players worldwide subscribing figure, but they did purchase the original WoW.
The easiest way to put it for somebody to (hopefully) understand is that when WoW dips down to 4-6 million subscribers in say maybe a few years, does that mean only 4-6 million were ever sold of WoW over the past decade? No.
Warcraft III, W3 expansion, D2 expansion, SC expansion, Orange Box, and a few others probably at least eclipsed the 4 million mark.
I imagine it is also difficult to get actual sales numbers. I recommend putting an additional note stating to the best of your ability these are the numbers that you could find, they do not reflect all of the possible games and/or numbers sold throughout the past few decades.