We have two children, but eight car seats. This fall there will be four cars in three different families transporting the girls. It's just easier than moving car seats every day in the winter. Especially a Radian. Those suckers are HEAVY. And the Evenflo Triumphs. We love them due to their easy adjustment straps, but they are SO. WIDE.
I had budgeted $300 for enough car seats for everyone and managed to score the (FOUR!) extra we needed for $95. The first to expire will be December 2015, so I won't have to worry about restraining devices for a while. Which nice, as it's not fun.