I used to use one pin on my older bow. I did fine with it, but I think I was actually doing more instinctive aiming than anything.
On my newer, old bow I have two pins I use. A 15 yard pin and a 20 yard pin. (my bow puts the 'arch' back into archery) I also have another pin that I don't use for shooting... it's not even in line with the others. I'm just too lazy to remove it. Besides, I never know when I might need an emergency toothpick or something.