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...