Two is the most for me. Three is pushing it. At one time I did a 3.5e, 4e, and Spycraft 2.0.

The 3.5e eventually died, DM went on to being a player somewhere else, haven't seen him since. The 4e was part of the encounters program, but was a custom campaign, due to work I can't go any more. The Spycraft 2.0 campaign ended with everyone being killed by another player, which everyone agrees was a **** move.