If you have a boneless pork shoulder and you figure the average person will eat a quarter pound (Some will eat less and some more, so on a average per-person) 250 people would eat approximately 62.5lbs. (250 People/1 quarter Pound = 62.5Lbs). If the estimate is one-half a pound per person then the total would be 125 lbs. 250/one-half pound=125). If the shoulder has the bone in then this must be modified for the estimate of the weight of the bone but is would probably me nominal to the overall calculation. If the pork shoulder is not tasty you may not need any on the other hand if it is exceptional you may need more, so expect leftovers and you will probably have none.