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@birdofafeather: We bought 3. I drop LO off at daycare, daycare takes her on outtings, and DH picks her up. All I can say is look for deals! I bought a Safety 1st Alpha Omega for $40 at BBB. It was originally just over $100 but was the floor model (not damaged at all, just out of box) and I had a 20% off coupon. The other two I bought on NBD. One was the http://www.nobetterdeal.com/shop/en/nbd/kids---baby/baby/car-seats/safety-1st-onside-air-protect-convertible-car-seat-b004ebux8k which looks uncomfortable from the pics, but is actually quite comfy for LO.
I also bought an Evenflo Triumph on NBD for about $32, gave that one to daycare to use.
My LO is 5 months old, but she's 22 pounds and her carseat was another 8. That's too heavy for us to carry in one arm, so we transitioned her this weekend.

From the standpoint of a Car Seat Technician I will say one per car is ideal. This eliminates the possibility of incorrectly installing it because of the frequency that you are doing it (its easy to overlook a step or get lazy when you are doing it on a daily or 2x daily basis.)

My daughter is 10 months old and I am 6 months pregnant so we will be transitioning her into a convertible soon to have the infant seat for the baby. When we do make the purchase we will be buying two convertibles- one for each car. Best of luck making your decision!