Is there ANY cooking method that doesn't shrink a brisket? I've braised, boiled, roasted, baked and
smoked them suckers and unlike many other cuts of beef, they shrink at least 50% EVERY time.
I think Karen's guesstimate is pretty close. If you're smoking or grilling you're gonna want to have a nice, thick fat cap on top. You have to take into account the fat to meat ratio, the amount of fat which you'll carve off and discard and the shrinkage factor.
Given the "1/2 lb a person" (generally the accepted portioning rule,) an 8 pound brisket (if there actually IS such a thing. Whole, untrimmed briskets run 10-20 lbs, halves are . . . well half that) would come out at least 4 lbs shy of whatever would be necessary to feed 16 adults.
At least any adults I know.