If you are serving a lot of other stuff (potatoes, stuffing, vegetables, dinner rolls, etc.) then two or three 20-pound turkeys should do it. They usually recommend planning for half a pound of turkey per person.
A very good question, I would project that you will need about 6 ounces of turkey per person. That equation allows for bones and skin weight etc. So if you were to feed 75 people I would go for either 2 18 lb turkeys. Or a 32-40 lb turkey