My daughter has 5 seats installed in cars, Me, DH, Grandma, Grandpa, and now her uncle has one too. The one in Uncle's car was her extra that sat at home, just in case. She goes to her grandparents' house once/week, but I leave a seat in each of their cars. I am a tech, and good at installing my seats, but once they're in I don't like taking them in and out. Too many chances to forget something. Also, because I'm very particular about the carseat usage, I don't want to have to trust someone else to install her carseat in an emergency, even if I have taught them. Anyone that would take her in an emergency now has a seat in their car.
The kids I nanny for have 3 seats each (and there are 3 kids). They each have 1 in my car, Mom's car, Dad's car. They have a few extra backless boosters for the big kids (6 and 9) in the basement, and for any friends they may have over. I also have 2 extra high back boosters, and 2 extra backless boosters for them at my house. The kids don't often ride in Dad's car (he works alot), but they all have a seat there anyway. The baby is 12 months, and she's only been in his car twice, but we feel he still needs a seat installed there. They have no family or close friends in the area, so the 3 of us are the only people that would drive the kids, even in an emergency (I live 15mins away, their closest family is over an hour away).
I'd suggest having a carseat in the cars your child may ride in.