Before getting all mine down to the basement, I had 7 on the same circuit, along with a sub and a popcorn machine. I suspect my builder cut some corners, because it you went to take a leak in the bathroom in the next room, the lights would dim when someone would hit the flipper buttons.
I would be money the bathroom lights were on the same circuit.
With that said, I never had and issue. I did however plan for 4 games on each 20A circuit when I did the basement. It's probably way overkill, but my wife wants a damn skeeball machine so who knows what I'm going to put down there.