Maybe I should add that it is very rare that I look at one position for several ours, but it happens and when I can't deside I can look at it again the next day. Sometimes I think about the position while walking to school and such. That flexibility and focus is hardly done with many games. I try to keep myself at a maximum of 6 games, but having joined tournaments over a year ago that will not be possible for too long.
I love how the brain processes information in the background. When I have a tough decision in a game and can't decide after a few minutes, I just set it aside for a while. When I return to the game, it's clear to me that my brain has been analyzing the problem at some level, or maybe it just got rid of the junk files getting in the way. It's a good technique for crossword puzzles too. Works for all kinds of creative problem solving really.
more than 20 at the same time will affect your play. I think it's good to keep it below that number.