Okay, quantity first....@ 4 ounces per person, and with 100 guests, you'll need 25 pounds of serviceable meat. When you are doing a carving station, you have the option of carving from the breast or thigh area. For presentation purposes, you have to stop taking from the bird when overmuch bone is visible. It may seem like a lot of waste, but when you are carving for show, you really want to offer the nicest slices. It's for this reason that I make the following suggestion. Make just one turkey, and have it presented on a lovely, well garnished platter either in front of you, or at the beginning of the line. For the guests, use turkey breasts. If you have a restaurant supply store (like restaurant warehouse) you can find what is referred to as RTC breasts. Ready to cook. Usually 9 pounds each, it's two boneless breasts, wrapped in net or foil, that you can roast, either in the foil or out. Once cooked, you'll have lost only about a pound or so to water and fat. You'll have a greater yield and much easire slicing/carving. It's been my experience that most people will request white meat, especially when making sandwiches.
If you follow my advice/suggestion, it doesn't really matter how you cook the whole turkey, since no one is going to eat it. Think of it as a $30 centerpiece. When you consider the price of flowers or candles these days, a $30 centerpiece isn't unheard of.
The RTC breasts I mentioned are best cooked from a thawed state. The meat is jucier and frankly, they cook in less time. 350 is all that is necessary, covered and you have to get them to 160. You don't have lots of time to get them back to 40 degrees; what we usually do at work is cut the breasts in half and wheel them into the walk in, temping them every hour to make sure they are coming down quickly enough. You mentioned lots of ovens, which is great, but do you also have lots of refrigeration space? They'll cool quicker if you take them out of the original pan you roasted them in.
If you still want to go the whole turkey route, I'd roast for the entire time at 350, covered till the last 45 minutes. You do have the same problem, though, with cooling. Conversationally, it's easire to rewarm smaller breasts than the entire bird when you are ready for service.
Finally, a 20 pound turkey will only yeild about 7 pounds of breast meat and perhaps another pound or two (at the most) of thigh meat. With possibly 100 guests you'd need 5 20 pound turkeys, leaving no room for error. 6 would be a safer bet. If you can only find smaller turkeys, you'll need more...
How can we sleep while our beds are burning???