Hmmm... two on each foot (one sitting on each ankle tickling toes, the other facing them tickling the rest), one kneeling behind me tickling my bum and thighs, two standing behind them tickling my underarms, two people kneeling in front of me tickling all of my front. I'm not sure what position I could be in to let them all fit in, but my new tickling fantasy is a rainy afternoon in with 9 ticklers!
I was tickled on a sleeper train by four of my friends once, but that wasn't for very long at all so I can't compare it too well with being tickled by two or three people. I think the biggest impact is made once you have two people tickling completely different parts of your body in totally different ways, say one on each foot when they're spread wide, or underarms and tummy. Maybe it makes it more difficult for your body to adapt, but I have never ever ever had any diminished sensation when being tickled by more than one person, but I have when a single person has concentrated on one spot for too long.