Honestly, I wouldn't use over 3. Maybe even go as low as two.
The kind of games that I would play on such a setup would be sort of parlor games. Maze games like pac-man and classic vertical shmups. Donkey Kong.
When I think of a coffee table I think of a sort of refined atmosphere. Simple games that can be played for high scores in a few minutes. Almost all of those games use one button, a few use two.
One joystick, one button, and one spinner?