Actually, I'm the one who suggested (and implemented) making the Demon Lords be summoned by rituals. While I could have had one ritual per demon lord, I just made 1 that summons a random demon lord instead. I set the world limit to 7 (the total number of demon lords) and the team limit to 1. Of course, as the infernal civ is not on the same team as who summoned it you end up having each demon lord bring in another demon lord until they are all present. I had a block stopping demons from summoning other demons, but then someone pointed out how he thought it was more appropriate to allow it so I let it slide.
Technically, completing the ritual won't do anything if the map has reached the maximum number of allowed civs already.