I usually carry a gun with two shells in it and two in my vest. When I'm away from home I load the gun with three. Strange place...you just never know what you may encounter I guess.
The two in the vest are mostly for if I forget to load the gun. (Yes, it's happened)
When I carry my single shot, I have one in the gun and an extra in my front pocket for quick access in case a follow up is needed.
I've had to shoot a couple of birds a second time, but it's rare. If I see that head up, I always shoot again since I lost one many years ago.
I've never come back from a hunt with no ammo and I don't see the need to carry a bunch of extra ammo and that weight. I did try carrying a third extra round, a heavy pheasant load for snake medicine but since it looks like my turkey shells I got it mixed up once and don't do it any more.
How many shells do you carry when you hunt?