I've heard that if you have too many for the hen to adequately keep warm at once, then they can all be compromised because each egg will get rotated out at some point during the hatch. After looking around for a few different figures, I played it safe and went with 12 eggs for one hen as a rule of thumb (regular size brown eggs, regular size large hen). I had 11 hatch both times. I suppose it pays not to get too greedy... 
As for turning and such, you don't have to do a thing. It's hands-off once they start incubating--another reason I love doing it this way. No worrying about temp or humidity or turning or not turning--the hen does it all, and much better than I ever could with even the best incubator set up. Ah, nature's perfection...
Edited by sky the chicken man - 4/3/12 at 3:39pm