Depends which child.
DS1 is currently on his 5th carseat and this does not include ones I bought and had to return due to not fitting in our vehicle. It also does not include doubles of the same seat because we have one for each vehicle. He keeps outgrowing seats by height long before weight.
He's been in...
Evenflo On My Way Position Right V
Evenflo Triumph
Evenflo Chase
Britax Marathon
Sunshine Kids Radian
He's outgrown every single one of those by height with the exception of the Radian... that's his current seat and when he outgrows it, he will go into our Parkway. So that is six seats for him.
DS2 I got smarter with... he's been in...
Graco Snugride (20lb, 26" model)
Britax Marathon
He will most likely need to go into the Radian as well after outgrowing the Marathon in height, but we have a long ways to go before that happens, he's still RFing! So counting a booster, that will be 4 seats for him.
I'll be even smarter when our third shows up!
Our third will go in...
either a Graco SS1 OR Chicco Keyfit30
then into a Radian when they outgrow their infant seat.
Counting a booster, that will be 3 seats for #3. See, I got better!