About half of a turkey's weight is edible meat, so if you buy a 16 pound turkey, it will serve about 8 pounds of meat. Most servings are around 3-4 ounces per person, so each pound of edible meat will serve 4 people. Here is how you would calculate it out:
120 people / 4 people per pound = 30 pounds of edible meat. So, you will need approx. 60 total pounds of whole turkey.