At the summer camp I attended as a yoot in the 60s, there was a game known as Mass Soccer. The chief feature of this game of games was that any number could play on either side. Kids being what they are, this meant that the entire center of the field was occupied by a permanent clot of forwards of all shapes and sizes, with a relative handful of backs whose chief task was to assist the goalies when a ball occasionally escaped from this huge scrum.
To keep things interesting, any number of balls could be in play at once. To keep things fair, each side was entitled to as many goalies as there were balls — when a ball went out of play, one of the goalies would do the usual thing to bring it back in while the game raged on around him. To keep things safe, the footwear rules were strictly enforced: no shoes allowed, socks required.
It was a hell of a lot of fun.