They developed WoW with an expectation of 400,000 subs total in a best case scenario, it went just a little better than planned.
Now, that would not support the current infrastructure, however, with sub closes, they downsize on infrastructure, so the cost goes down as revenue goes down.
I'd say 300k to 1 Mill is needed to keep content updated at a decent speed, but that would bring a hell of a lot of server closes, staff lay offs and much longer and less customer support etc.
Tbh, they could make it work with 100,000k with a skeleton staff and basically no updates, but that wouldn't sustain players interests for long.