When after a hand is over you look at a screen and say "holyshit, this table is only playing 3 handed", that's when you know you are playing one too many tables. Or when a table pops up and you are still involved with a hand post flop and you aren't sure why. I think 2-4 is optimal for six max. A lot of fr players can play 100 at a time since they are seeing way fewer flops. I would hate to see how stressful it gets when you get like 5-6 good hands at a time.