How much turkey should I plan per person?
Unless you're feeding 20, chances are you want the smallest bird they have. It doesn't really matter if you should have 1 lb per person or 1.5 if you have 8 people and their smallest bird is 12 pounds - and I'm willing to bet that's the case.

That said, I generally allow 1 lb per person and don't count the smallest children (say, haven't started school yet.) I also tend to send a lot of leftovers home with other people. Our feasts are very veggie heavy, but as I mentioned above you may have less control over this than you think.