Well, my son has 4 different car seats. One in my car, one in my husbands car, one for my dad & one for my MIL (they each watch him 1 day a week & each bought themselves a car sear). All of us have a base for the infant seat, and only 1 infant seet for the upcoming baby.
When he grows outa that, and I'm assuming my son will still be in the convertable seat he is in now, everyone will get a 2nd convertable seat for the new baby. That will total 8 seats for 2 kids.
I don't like playing the "switch the seat between cars" game & he is in all 4 cars enough to warrent so many seats.