As a skeet shooter, there is allot of consideration on this--as mentioned, we just need a chip out of the bird for a 'hit', nothing else. Nice to see them dust, but its not required. Since shot is loaded by weight, the size of the pellet will determine how many pellets you get in a shell--the link below will give you an idea on that:
http://www.shotgunworld.com/amm.html
Obviously, the smaller the pellet the less it will weigh, and the more things like wind will affect it. Additionally, wad and choke selection will play into shot column density and compression as it leaves the barrel. However, the smaller the pellet, the less mass it will have, and the less energy transfer to the target. I've seen 9 shot out of a 20 gauge just "push" the clays. I generally run 7.5 as the heavier pellet is a better trade in my opinion than more pellets.
Another factor to consider is the velocity of the shells. A higher velocity/dram eqv. will give you more punch against steel than say a low-recoil target load. I'm still a begginer 3-gun shooter, but my target loads haven't had issues on any of the steel--they're Federal GM cases, 1oz 7.5 shot, at running about 1180fps. Most standard loads run around 1200fps, with handicap running 1300fps. I also have some cheapo wally-world Remington cases that suck for reloading--so I'm just letting them and the Federals fly (for skeet I'm going with Rem Gun Club hulls).
One last thing if you're reloading: chilled vs magnum shot. Chilled is pure lead, magnum is a alloy. Problem with magnum shot is its lighter than chilled, and if you're using a Mec press for example, the 1oz shot bar is bored for chilled/pure lead shot so your load will actually be a bit light if using magnum. Easy option is to just ream the shot bar out till it drops the specified weight of magnum--but then you'll need to adjust the wad compression due to a bit more shot being dropped (again, everything is from you only had lead shot).
