I usually bring along a box of 20 on a trip to drive-in areas, 40 on fly in or remote spots. Carry a dozen in farming country, 20 in the deep bush.
One memorable deer hunt in a wilderness area actually required 14 shots to kill a deer. It was a very unusual situation, don't expect to ever repeat that. Most times it takes one shot. But I like to carry more for re-sighting, emergency signalling, possible varmint or small game shooting, etc.
I just returned from a safari in South africa. 7 head of plains game with 7 shots, and then there was the Eland. Well, the Eland required 8. Dang.