Wow I honestly thought I'd be in the minority, thinking people would answer 8+.
I can only play 4 tourneys max, also. Even then, things would start getting too fast and too distracting for me to make quality decisions (the 15" computer screen could be a factor, too). 2-tabling is just Goldilocks, but unless there are overlapping tournaments, I'd much prefer to focus on one tournament, one hand, one villain at a time. Easier to read the table dynamics and to pay attention to opponent's tendencies, even without showdown.