I don't think the number matters that much. I think the title MMO suggests that it's a game that is played in an open world with other people, with objectives that happen in the open world. It's also continuous.
So, MMO could really just be renamed Open World Multiplayer Game, OWMG. I think "massively" as an adjective also refers more to the type of interaction. That is, massively means that most of your interactions are with other players, and that you're always or almost always in a state of being in a multiplayer mode.
It's not hard to imagine some indie MMO that only has 100-200 players all on one server. Like, if WoW had 200 people playing total, it would still be classified as an MMO.