My answer is 'as much as you can afford'. But the actual formula is 1.25 pounds per adult.
Number of guests x 1.25.
That means that if you are expecting 12 guests, plan for a 15-pound bird. If you have kids coming, simply cut their portion in half. So if it's 12 adults and 12 kids you go to 15 pounds for the adults and another 7.5 pounds for the kids. The kids usually go for the rolls and side dishes. Pro tip: If you have a large group this year, try dividing your turkey number by two and buying smaller birds. This will allow them to cook faster and more evenly.