I would agree with Andy as he is proved time and again that he knows his stuff, but... We make two twenty pound turkeys every year and normally have 6 to 7 adults and a couple kids. The problem we have is that some of those who come (its complicated so we can't leave them out) seem to take half or more of whatever is left over, and usually all white meat. There are also at least three that won't touch dark meat. Fortunately, nobody in the group except for myself and my daughter know about the oysters, and so we sneak them for ourselves. Simple greed insures that those who paid for the food, and cooked it, get very little left overs. It's discouraging. But as I said, it's complicated.
I don't know what kind of manners your guests have. I hope they are better behaved than those I have to deal with. But two birds will assure you of left overs if you want them. Also, I never use the pop-up timers that come in the birds as they don't activate until the meat reaches 180' or so. When the final temp is that high, the bird is overcooked and very dry. It took me too many years to figure that out.
Invest in a cheap meat thermometer, the kind you can leave in the bird while it's roasting. Insert it into the bird so that the tip is buried in the deepest part of the turkey breast, but no touching the bone. Rub the skin with butter or cooking oil, or olive oil as you prefer, and salt the skin with kosher salt. Place the turkey onto a rack, in a shallow roasting pan and pop it into a 400 degree oven and cook for about 12 minutes per pound. Then check the thermometer every 15 minutes after that until it reads 155' F. Remove the bird and let it rest for twenty minutes. To carve, cut the legs from the body at the hip/thigh joint, and remove the wings at the shoulder joint. Cut the entire breasts away from the carcass by sliding a boning knife along the breastbone and following the bones until the breast is removed. Then slice against the grain of the meat to provide everyone with skin, and very tender and juicy slices. Arrange the breasts side-by-side on a serving platter, with the wings and legs placed at the sides, approximately where they would be on the whole bird.
You can cook the turkey two days ahead if you want, giving you time to do both in the oven. Just make sure to seal the meat in airtight containers, with turkey broth to keep it moist. Then, just reheat on Thanksgiving. If you store the turkey in aluminum pans, you can put the pans directly into the oven and cook them for about 40 minutes and be ready to serve.
Reserve some of the broth for gravy. Boil the necks, livers, and gizzards to make broth for the dressing. If you know anyone who likes liver, take the cooked livers, and place into a blender or food processor with butter, a bit of salt, a bit of garlic powder, and a little onion powder. Process until silky smooth and you have a very tasty liver pate'. The hearts and gizzards are muscle tissue and can be diced and put into the dressing.
And I whole-heartedly agree that others should bring something, like mashed rutabeggas with brown sugar and butter, veggie trays, mashed potatoes, whole wheat dinner rolls, deserts, jello salads, etc.
To make things even easier on you, you can purchase leave-in meat thermometers that are attached to a temperature gauge that stays outside the oven. You set its alarm to go off at a desired temperature (155'F) and walk away and do other things. Then you just don't have to worry about the bird. You can watch TV, or prepare side dishes, or play with the cat... When the alarm goes off, remove the bird from the oven and let it rest.
Seeeeeeya; Goodweed of the North