Two is no problem, but depending on how deep I get and if one or more is a faster structure.... Well, 3 and up can cause mistakes, at least for me. I've totally accidentally folded straights and such when a window focuses it's attention and i'm already clicking for another table. I just scream and move on, but i've learned to at least try and avoid doing too many tourneys. When i'm in that situation, it's usually because I didn't expect to go deep in all 5 or so that I started. Ya figure you're gonna bust out of most, so why not right? Not a bad situation to be in, I just hate it if I make a costly fold or call for that matter. Have also accidentally hit call on some raise that I would have had absolutely zero intention of ever calling (nice if it works out, don't like looking like *that* donkey tho) with the shit cards I happen to hold at the time.
So yeah, I do play more than one, but I find I'm much more consistent and tend to go much deeper if I can stick with 2 max.