I just did my first "on my own" shrimp boil last week and had the same questions. I was boiling shrimp for 7 and had the usual sides (corn, potatoes, garlic, onions and sausage) included in the boil. I bought 15 pounds for 7 people ended up with almost half left over.
I'd say a pound per person if you have sides and are drinking a few beers. The guys will eat possibly a little more but a woman or two stopped after 10 shrimp.
P.S. we peeled the rest while drinking beer and enjoyed them cold the next day for lunch AND dinner.
This post was edited on 6/11 at 10:51 am