Perhaps it might be worthwhile to start by listing the nominal diameter of the various common sizes of shot as manufactured in the U.S. Here they are, courtesy of the Federal Ammunition Catalog:
BBB .190" (4.83mm), BB .180" (4.57mm), 1 .160" (4.06mm), 2 .150" (3.81mm), 3 .140" (3.56mm), 4 .130" (3.30mm), 5 .120" (3.05mm), 6 .110" (2.79mm), 7 .100" (2.41mm), 7-1/2 .095" (2.35mm), 8 .090" (2.29mm), 8-1/2 .085" (2.16mm), 9 .080" (2.03mm).
Here is the average pellet count in one ounce (28.35 grams) of 3% antimony, 97% lead shot of various selected sizes, courtesy of the Winchester Ammunition 2004 Product Guide:
BB - 50, 2 - 87, 4 - 135, 5 - 170, 6 - 225, 7-1/2 - 350, 8 - 410, 8-1/2 - 497, 9 - 585.
Here is the average pellet count in one ounce of steel shot of various selected sizes, also courtesy of the Winchester Ammunition 2004 Product Guide:
BBB - 62, BB - 72, 1 - 103, 2 - 125, 3 - 158, 4 - 192, 5 - 243, 6 - 315.
Target Shooting Games (lead shot)
16 yard Trap - 8, 8-1/2
Handicap Trap - 7-1/2, 8
Skeet - 9
Sporting Clays - 7-1/2, 8, 8-1/2
Live Pigeon - 7-1/2
Upland Game (lead or tungsten alloy shot)
Turkey - 4, 5, 6
Pheasant - 5
Chukker, Grouse, and Partridge - 6, 7-1/2
Quail - 7-1/2, 8
Dove - 7-1/2
Rail, Snipe and Woodcock - 7-1/2, 8
Rabbit - 6, 7-1/2
Squirrel - 6
Waterfowl (steel shot)
Geese - BBB, BB, 1
Ducks (over decoys) - 2, 3, 4
Ducks (pass shooting) - BB, 1, 2
Waterfowl (tungsten alloy shot, Hevi-shot)
Geese - BB, 2
Ducks (over decoys) - 4, 5, 6
Ducks (pass shooting) - 2, 4