There are 8 cups of cooked pasta in a pound. If you are serving as an entree then plan on 1 1/2 to 2 cups per serving. For a side on a buffet plan on 3/4 to 1 cup per serving. This is for sauced pasta, if you are doing baked ziti then it will be different.
When I am doing a buffet with pasta and 2 other sides I normally get 9 to 10 servings of pasta per pound.