How many mussels (average from Maine) are in a pound? How many pounds for 12 people?

I'd also go back a step and try to find P.E.I. mussels, but maybe (just maybe) I'm biased. ;)

Also depends on whether it's a starter or main course. For the latter, I'd definitely figure 3/4 pound (9-10 lbs total) - especially since there are invariably some dead ones you'll end up tossing when you clean them.

There are usually 20-25 mussels in a pound, and you'll want 1/2 to 3/4 pound per person.