20+ Players

The most people I have ever DM'ed at once was around five or six. But I have heard and read about campaigns (even ran by people like gary gygax) in which there were multiple parties of chracters adding up to twenty or more players sometimes not even fully aware of each others actions so they could enter a dungeon after another group had already gone through.

This sounds very intriguing to me but I haven't had any playing experience remotely like that. I doubt I could find and manage 20 players in my area or find a group like that but maybe there are ways to simulate such a campaign. Here are the ideas I have had to try and simulate it or make a campaign world more alive.

1. I could run a group of characters into a dungeon myself, like a solo adventure and leave behind their actions for players to discover.
2. I could use official tsr adventures and look for campaign logs online and incorporate some of what they did into the adventure.
3. I could make some tables too randomly allow encountering or rolling up the effects of previous adventuring groups.

Obviously this won't be the same as actually running multiple groups but it could be interesting. Does anyone have any more ideas?