I've been known to play 24 at a time. I play mostly at the micros so it's basic tight abc. The problem with playing so many is that you often have limited time to make decisions. This, however, can also be an upside as it forces you to make decisions quicker. Obviously, if you rigorously review your game then your decision making process will improve drastically. The main problems I had with multitabling were when I had four or five final tables going at once. I used to have to go into sitout in the multitude of other games I would have playing. Nowadays, my playing system consists of 4 tourneys, 4 sngs, and the rest of my tables are cash games. That way, when approaching final tables I can finish sngs and not start a new one and close cash games at will.
Another point is that, when I started multitabling, I looked into the effects of loss in expected ROI and started with 4 tables and added 2 at a time until I was comfortable with that many. I remember when I first started 8-tabling how hard it was. It is doable though and with tools like placemint it's very manageable. I have a four 19" monitor set up and can display 16 tables per monitor.