How many bullets are one "round?"

One bullet equals one round... May go back to when shot were actually round, as buck shot still are, but buck shot is many large shot in a single package...

Technically, "shot" in this case is plural. If you take a typical shotgun round apart, you'll find several round pellets inside that can differ in size depending on what type of shotgun round you're looking at.

Yes; until recent times it was possible to get round ball projectiles for shotguns... Now, solid shot, or slugs as they are called resemble the mini balls used during the civil war, which were open in the back side, and expanded into the barrel, but the shot gun slug does not do this. There is a plastic sort of wadding that protects the barrel from wear, and on rifled, "deer slayer" barrels, the wadding spins the projectile, but falls away after the slug leaves the barrel... So yes, in olden days, round balls were the projectiles used in both pistols, and smooth bore muskets, and even rifles... A better understanding of the flight dynamics of bullets gave a different shape to them, but they are still referred to as round, and I think that is due to the shape of bullets originally, but the sequence of loading and firing, beginning and ending as it does in the same place, a cycle, may well be considered a round, but I think this conclusion is unlikely...