Hi Maya and welcome to the smf ...... i've read on a post recently that you can expect get a 50% to 60% yield from a butt. So 4 - 5 lbs out of an 8 lber. At roughly 1/2 lb servings per person (generous to allow for seconds) you're looking at 4 - 5 8lb butts. If in doubt go more then can always freeze any left overs for future dinners and sandwiches.
I only done my first butt a few weekends back and followed meows "basic pulled pork" sticky method posted in the pork section. That was the best pork i have ever done.