whatever works for you and what your comfortable playing. everyone has different styles and approaches to the game. Some people can play 10 tables+ mixed games with mixed # of players or some full some HU. I dont think i could ever do that and having to click my mouse every split sec for hours isnt something i care to do anyways.
I know if im playing cash tables i like to play 2 or 3 any more and my game falls off. 1 table just not enough action, i get bored and play too many
hands poorly. 2 or 3 has worked out best for me so far. I just make better decisions and i am still able to pick up the things players are doin in specific situations, even in hands im not involved in. When playing more you pretty much start playing your cards only and not paying attention to your opponents. Your also only able to see hands that you are playing and could easily miss out on important information from a previosu hand between other players.
In 9 player sit n gos though i will play anywhere from 4-10 and not really feel like my play has been affected. i think if i played less at a time i might profit at a higher percentage but only by a small margin. say i play 2 sit n gos with a 50% profit or 8 with profits of 25%. i would make twice as much playing 8.