The rule of thumb is about 1.5 pounds of turkey per person, to provide plenty for the dinner itself and leftovers.
For the turkeys we have available, here are some conservative guidelines:
Medium: serves 8 people
Large: serves 10 people
X-Large: serves 12 people
We love leftovers, but if you don't you can dial this down a bit. For parties larger than 14 you'll probably want more than one of these turkeys.