I'm not overly familiar with the subject matter I'm afraid, my multiplayer is done on a small scale (8 players), so hosting a server on a GM program is fine.
But the common suggestions seem to be to use something along the lines of Python or JavaScript.
Someone more knowledgeable on the matter can probably help you further, but it will likely involve you becoming familiar with another programming language if you want to make an MMO that uses GM for the clients.
I think many MMO's probably do use more than one server though. I don't know specifics, but an MMO likely doesn't have more than a few thousand on each server, and they'll be beefy servers.
If you are new to GM as well, I'd recommend you work on smaller projects before trying to make an MMO, they are among the hardest projects to make.