I think you will need just under half a pound of dry Elbow Macaroni to get one pound of cooked macaroni.
Most dried pasta doubles in volume when it is cooked. Generally it is assumed that one pound of dried (or freshly made) pasta will serve 6 as an appetiser or 4 as a main meal.
It is more accurate to measure pasta by weight than by cup so if possible, use a digital scale.
4 ounces of uncooked pasta (elbow macaroni, shells, penne etc) is equivalent to one cup of dry pasta which makes 2 1/2 (two and a half) cups of cooked pasta.
A little trivia for you: Did you know that elbow macaroni gets it's curved shape from each side of the machine it comes out of operating at different speeds?