I knew a family with 5 boys --- all teenagers at the time who owned a quite small 2 bedroom house --- the parents had one room and all 5 boys slept in the one small bedroom --- wall to wall beds. So really you'd only need 3 bedrooms for 15 kids --- less when they're little.
We have 7 children, the two oldest share one bedroom( it's really a den with pocket doors), the next four share a room, and the baby is still in our room. We are building into our attic for more bedroom space, a second bath, storage, more "living room" etc.
Big families are really not that expensive if you do it right. Most two children families spend FAR too much on name-brand clothing, name-brand food, new cars, high mortgages, cable TV with forty-seven movie channels, the list goes on and on.... I'm always amazed by how much money people make that claim they can't afford children. I wish I made that much money!
Cook from scratch, buy used, fix things yourself, grow food in the yard, don't go to the movies, don't pay extra for crap (cable TV) that doesn't help the family (cable TV) and one can raise a large family rather cheaply.
I applaud this family for their debt-free living and doing the Dave Ramsey Seminars for others. It's a great blessing to teach others how to really handle money.
And I especially applaud this family for raising future conservative voters!