If doing bone in pork butts figure on getting a yield of about 50% of raw weight. I find if people make their own sandwiches they use about 1/3 lb per sandwich on a normal sized bun. If your serving you might be able to keep it closer to 1/4 lb per sandwich. If you started out with 16 lbs of butts it should yield about 8 lbs and should get you 24-25 sandwiches but that would only be 1 per person.
The average is 1/3 lb. smoked pulled pork per sandwich. Its a good size portion for "Healthy Eaters". 25 people divided by 1/3 lb. is approximately 8 1/2 lbs. of smoked pulled pork. With a 50% yield your looking at about 17 lbs of raw pork butt. I would probably do a little more to cover the healthy eaters and make sure there were some leftovers because I don't like to run out when I am serving others. The sides of course will help cut down on the amount used for sandwiches. You may also get some people that won't put it on a bun. Also depends if you are talking all adults or Adults and children. Good luck!
If I figured 1 sandwich for a lot of my company I'd be in deep do cause they'd still be hungry. Personally when I smoke for friends I figure closer to 1/2 lb finished pork per person and that way I never have run out. Sometimes I'll give some people a doggy bag to take home if there's enough left and that never makes them sad
You be safe with the 1/2 lb per person. You don't want to be sitting there with a bunch of hungry folks. Remember it's quite a complement to the chef when they want more of your food. So keep them happy and serve them all they want. You can always freeze it and re-heat it.