The cars with more than 5 seats do exist. If cost is not an issue as you suggest, it is pretty feasible to have 4+ children. It is even easier if you are not aiming to have them born in a rapid succession. Older kids can ride upfront.
Just going from 2 to 3 kids shocked me at the cost increase, and we already had a minivan (hotel rooms sleep 4, so now we have to get 2 rooms or a suite). There are a lot of other things that go up a lot more than you would expect when you have more than 2 kids. Kids eat free is normally each parent can have one free kid.
Overall kids are not very expensive, and I'm glad I have them. However there are a number of things we can't do anymore. Family memberships are well worth it for us since most of them are one price no matter how many kids you have, and thus cheap entertainment for the rest of the year.