Lol, you still dont grasp the concept. Maybe I can help.
1" main line, 10 people in the house, 90 grains hardness, customer wants to maintain decent efficiency and regenerate approximatley every 6-7 days at 6 pounds of salt per cu. ft.
10 people x 60 GPD x 90 gpg x 6 days between regeenrations =324,000 grains system needed for this application.
So you would have no problem installing a 15+ cu. ft. single tank system?
A 4 cu. ft. system should work ok.