Unless this is going to be a serving line that is tended by servers dishing out exactly 1/4 lb portions to each person I would figure on at least a half pound per person and more if if it is gonna be an all day all you can eat grazing buffet or you are inviting a bunch of the moochers that I hang with. So by my twisted logic figure 50 folks at half a pound and you get 25 pounds of cooked pig meat. The trouble is that cooked pig meat has to start out uncooked and some where in the process you loose about a third of the weight.(eh maybe more, maybe less) So add about a third and you now need 37 1/2 lbs uncooked to end up with the 25 pounds cooked. Depending on the size of the uncooked butt, let's say six pounders, you will need six butts. But wait, there is more, what if the party is a hit? What if your cooking is as good as you think it is and every one wants even more? What if say like ten more folks show up? What if nobody shows up?
You may need a back up plan. I would figure on the six butts if the numbers are reasonably firm on attendance and be prepared to instantly package and freeze any pig that will not be used that day. I would also have nearby another 10lbs, or two butts worth, frozen as a back up just in case the crowd got a little bigger than expected. Have four butts pulled and or sliced and ready to serve at the sound of the bell and be ready to serve the next one or two as needed. If you are getting into that fifth butt you need to take a good look at things and decide if the frozen butts need to come out!
Alot really depends on the function or occasion. 50 folks at the retirement home is not the same as 50 Hells Angels. I would try to err on the high side. I just hate to see folks go away hungry, especially me! And like I said, unless you are doing the serving and controling the portions the meat will usually go faster. (You could find some smaller diameter buns