How many cubic yards is that?

Multiply length × width × depth with every dimension in feet, then divide by 27, because a cubic yard is 3 × 3 × 3 feet. If your depth is in inches — and it usually is — divide it by 12 first. A 10 × 10 ft area at 4 inches deep is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards.

27. It catches people out because a yard is only 3 feet, but volume goes up with the cube: 3 × 3 × 3 = 27.

Bulk material is sold by the cubic yard from a yard or truck; bagged material is sold by the cubic foot. A 2 cubic foot bag of mulch is 1/13.5 of a yard, so a yard is about 13.5 bags — which is why bulk is almost always cheaper once you are past a few bags.

Usually yes. Bulk material settles, subgrade is never perfectly flat, and running short means a second delivery fee that dwarfs the cost of the extra material. Ten percent is a common allowance on fill and aggregate.