Depends on the size of the potato. If you're using Russets, and I hope you are, if they are large Russets, plan on 1.5 potatoes per person or 1 per person if they are really large. If they are smaller Russets, plan on 2 per person.
A 10 lb bag of med sized Russets contains about 25 to 30
potatoes. Following sgsvirgil's 1.5 potatoe per person guideline
above, a 10 lb bag would feed 20 people. Which comes out to...
1/2 lb a person, again as suggested above.
And that sounds about right, after you boil, salt and milkerize them.
So 5 lbs would do the job, with a little selective portion control.