Honestly, I don't think there's any tabletop system designed for that many players. The only possible exception is Tunnels & Trolls, where (I believe) each side adds their dice together for one big roll, winners dividing the difference as damage among the losers. At which point the tabletop collapses due to hundreds of dice hitting it at the same time.
Originally Posted by Moritz
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)