It all depends on the clamps you're using and your string method. Wen doing mains always use two one per side. When doing crosses top down you can use one clamp if you use a starting clamp and have flying clamps or a glide bar clamp that will service each side. If you are doing a two piece 50/50 you must have two clamps for crosses.
I have the ability to drive people crazy, not sure if I was born with it or I learned it but I'm damn good at it.