At Cooking Light, we allot 3 ounces cooked meat per serving, which takes into account 25 percent shrinkage of meat during cooking (meats lose mass and volume while they cook as fat renders, moisture evaporates, and muscles tighten). So the formula is simple: four ounces raw beef per person, and you can extrapolate endlessly. If you're feeding four people, you will need 1 pound (16 ounces) of raw, trimmed beef tenderloin, which will yield 3 ounces cooked beef per person. A five-pound trimmed tenderloin will feed 20 people, and so on.
All that said, however, it's the holidays, and people may want to eat a little more during this festive time. To avoid being a Grinchy host, make sure to buy and cook a little extra.