I get it. It's confusing. So let me help you.
Step 1: Determine how many eaters are coming to your turkey party.
If you have less than 12 people, account for roughly 1 1/2-2 pounds of turkey per person, assuming you want leftovers. For smaller birds, there is less meat-to-bone-and-organ ratio, so you need more weight per person.
And for both sizes, if you do not want leftovers, simply select a bird slightly smaller than what the above formulas yield.
Step 2: Plan your cook time accordingly.
When cooking your turkey, it's standard to start cooking for the first 2 hours at 325˚F, then increasing the temperature to 425˚F for the remainder of the cook time.
If the turkey is between 10-18 pounds, cook for 3-4 hours, (3.5-4.5 if it's stuffed).
If your turkey is 18-22 pounds, 3.5-4.5 hours should do the trick (4.5-5 if it's stuffed).
But for your own safety, make sure the thigh of the bird reaches 165˚F before you remove the turkey from the oven. Don't skip this step—breaking out the meat thermometer is definitely worth avoiding foodbourne illness for you and your guests.
Wishing you a delicious, well-portioned, and illness-free feast.