Are all fifty going to eat roast beef and are all fifty adults.if they are all adults (big eaters) and all are going to eat roast beef(and nothing else) then I would say at least fifty pounds. Now if your going to have other foods/sides/drinks/breads/salads/snacks then I would say about 38 lbs. 3/4 lb per person.any left overs could be taken home/made into subs.just my opinion.
