What size turkey do you need to feed 15 people?
According to the website mentioned in the Related Links, the rule of thumb is to pick a bird that weighs 2 pounds more than the number of people you are feeding, e.g., a 10-lb turkey would feed 8 people; and a 15-lb turkey would feed 13.
To feed 250 people you will definately need more than one turkey. A 20lb (10kg) turkey will feed ten people, so you will need 25 x 20lb turkeys to feed 250 people. Very few catering venues or kitchens will be equipt to deal with the amount of cooking involved in roasting 25 turkeys, so it might be best to think of other catering options.
How much turkey you need for each person depends on the size of the individual turkeys. Turkeys under 12 pounds have less meat on them so you would need to allow 2 pounds per person. For turkeys over 12 pounds, you should allow 1 1/2 pounds per person. For 70 people you would need between 105 and 114 pounds of turkey.