i average out at 15 sandwiches per butt and it usually works.
i estimate at 4 ounce sammies. An 8 lb'er yields 4 lbs cooked, which is 16 sammies per butt. When i buy, i buy 15 lb packs and better. (boneless).
for 300, I would buy 20 butts. (300/15 sammies)