I believe the current world record holders are Tim Kulinski and Baron von J. Both have run thrilled-packed Pulp Alley convention games with 8 full leagues on the same table.
Personally I draw the line at 6 full leagues - if most of them already have a decent understanding of the game.
However, if I'm doing a group of new players then I prefer to draw the line at 4 players. And I may even reduce each league down to 1 Leader, 1 Sidekick, and 1 Ally.
Also, if there are at least 2 other players then I will simply GM their first game - rather than try to play and teach at the same time.
The Pulp Alley mechanics do tend to be much more engaging for all the players - because you never know when you may steal the Initiative, are forced to activate one of your guys, want to counter-attack against someone that is shooting you, and so on...
Pulp Alley demo games can be relatively short, so you can always run back-to-back games for two groups rather than trying to fit everyone into one game.
HAVE FUN... and take photos!
