Yes, and no.
We have two convertibles in my car, one convertible and a bucket in DH's car. Since we have an extra base and DS fits the bucket, I guess that makes the convertible DS is using in my car an extra. But it's currently in use, so it's not spare. Then we have two convertibles in SIL's car. She takes the kids infrequently enough that she doesn't really need seats for her car, but I'm not always available to install and DH and SIL are both okay, but I'd much rather check their work. SIL doesn't strictly need seats at all, she could stay home with the kids, but since she regularly watches my kids for free, I am happy to provide seats to make it more convenient for her.
So depending on how you look at it, we have no spares, or one spare, or two spares, or three spares. I'm hitting up Targets today, trying to score something on clearance other than a PINK seat, because I'm looking ahead a few months to when DS outgrows the bucket and we don't have a seat for DH's car.