Like Russell says, there are several factors including types of sides (how starchy they will be), breads, etc to be served.
Brisket is a fairly rich meat so that plays into it as well. When I do a brisket, I plan on almost 50% shrinkage from raw til finished.
If you play the safe side, I'd have 8 ounces of cooked meat per person...and hope like crazy I'd have some left over to munch on the following week. Brisket just seems to get better the week after!