question about cars and car seats

Okay so I have an 8month old and one on the way. How does everyone with big families even do it when it comes to cars and car seats. Right now we have an SUV that has two car seats in it. We are planning on getting another SUV so do we put two car seats in that one too? I just think that is the easiest (yet most expensive) way to do it in case we are unsure of who is picking up the babies in what car etc. I know there has to be an easier andcheaper way. What do you all do? And what happens if you have 3 or 4 kids who are in car seats ?? Yikes