Yep, you'll need 2 10lb briskets. You could probably get by with one if you add 5-6 links of sausage. I did 3 for a group of 30 2 weeks ago and had about half a brisket left.
a pound a person. It kind of depends on what you have for sides..if you make some good butter and onion boiled potatoes, pintos, coleslaw, corn on the cob, etc, you might not need a lot of brisket.