I don't often play more than 3 at a time, and not as many if one of them is a long haul mtt. If I'm playing sng, I normally play for 15mins or so before starting the next game, that way i don't have to focus more on more tables before the 1st one is finished (eg fifty/50, normally only lasts about 25-30 mins).
if i have more than 3 tables running and its getting close to bubble time on all 3 of them, things can get to be a bit of a chainsaw juggle, with a lot of people playing desperate; which is a time i should be maintaining more focus.
I occasionally try more than 3, but it's not worth 3x the fun value of playing just 1 table, if that 1 table is being good to me.
And if i'm not having fun on just one table, then I'll take a break for a bit.