For mini/slider buns, you're probably going to need 2 - 3 ounces of pulled pork per bun.
Depending on how you're cooking your pork (smoke or braise), the yield is going to vary a little bit, but in general you're going to get about 40-60% yield after trimming, shrinkage from cooking, and probably some waste from pulling. Buying bone-in pork butt will mean a lower yield, closer to that 40%.
So for instance, if we assume you'll get an average yield of about 50%, that means that buying an 8 pound pork butt will only give you 4 lbs of edible meat.
If each sandwich is 2 ounces, you'll get 8 sandwiches from each pound of pulled pork. That means you need 2 pounds of raw pork but in order to get 8 sandwiches.
Hopefully that makes it easier for you to figure out how much pork to buy, based on how many sandwiches you want to serve. Let's say you want enough pork for 200 sandwiches. 200 divided by 8 = 25 pounds of finished pulled pork. So you'd need 50 pounds of pork butt. If you want more or less sandwiches than that, just do the same math.
Hope that helps!