As Poker Orifice say, it really depend on your level of experience. And also your equipment. 12 tables on a laptop quickly become very confusing. I will say though, that in general 9-max MTTs tend to play incredibly slow, so it really does not take much skill to play a few of these simultaneously.
Each table see maybe 50
hands an hour, and out of those you are only involved in 10, if your VPIP is 20, so its 6 minutes between you needing to do anything other than pressing "fold" preflop. For me boredom therefore quickly become a factor, if I only have one table running.
I dont play endless sessions though, so if I run deep and reach a final table, often that will then be the last table still running, or maybe I have two tables left at that point. And that is ok, because it allow me to concentrate and spend more time observing, when the money really matter.