Other contributors have said "How do you roast a hog?" is the same question as "How many people will one hog feed at a roast?" If you believe that these are not asking the same thing and should be answered differently, click here
Taking into consideration that the pot roast will be served with other sides, such as mashed potatoes, veggies, and whatnot, the average person would probably eat only a quarter to half a pound, so...
All together you would need about 7.5 lbs. So it would not matter how many rump roast as long as they weighed 7.5 lbs in total.As to proper portions per person. 8 1/2 to 9lbs would be best should...