mean do they mean 20 online at once but can have more than that in accounts/players?
do they mean 20 people per game(assuming a host kinda thing)?
do they mean out of everyone who plays the game anywhere ever there can only be 20?
That means 20 users online at the same time.
Yes.
Not necessarily. Depends on what you mean by game, depends on your set up.
It means 20 at the same time.