if you have multiple kids, how many car seats do you have?

Once they are out of the infant carrier. Do you have one car seat for each car for each child? We have two convertible car seats for our one child, one for each car.  Now we have baby number two on the way.  Seems like a huge pain to be switching 2 car seats between my car, my husband's car, and the nanny's car so thinking we'll get two more. Is this what other people do?