I hit a rock and it destroyed my tire and damaged the (steel) rim.
Should I replace 2 rims (matched pair), or is it ok to replace just the one damaged rim? Is the car going to be balanced? It is the rear wheel and my car is a FWD sedan.
Looks don't matter because they're steel rims covered by plastic wheel covers.
If it's not badly-damaged, you may be able to repair the rim instead (or have it repaired by someone). If it's just dented or slightly out of shape, a firm hit with a heavy hammer and a round-end punch could put it back in shape. Put some paint on the scratch and off you go. Have it checked/balanced by a tire shop; if it does not come back with an excessive quantity of weights, you're fine. Usually, the shop will tell you beforehand if something's wrong.
If you notice a crack on the wheel, junk it. Give it to someone that could re-use it as scrap metal.
If you end up buying a new rim, since it's at the rear and your car is a FWD, you should not have to worry too much about its size or its weight. If it were on the front, a significant size difference can damage the differential on the long run, and a significant weight difference could affect acceleration and deceleration due to the different inertia of the wheels. Just make sure that the size with new tire is close to the wheel on the other side.
From what you describe, it's unlikely that you will need to replace two steel rims rather than one. If you get an equivalent diameter, width and construction it shouldn't be a noticeable difference.
You should examine the other tire for wear, though. If it's diameter is going to be significantly less than the replacement in the other side, that may mess up the driving feel. This is less of a concern on the rear wheels of a front wheel drive car (it's an open differential in the rear so different rotations won't cause major trouble).
Worst case: you might decide to replace the destroyed steel rim and both rear tires.
You should be able to replace the one rim on its own, however depending on the wear of the tyres on the same "axle" i.e. the one opposite it, you should maybe consider replacing both tyres to maintain even grip characteristics.
As long as you replace the rim with one that is mostly similar to the other rim the balance should not be affected at all. Tires (and rims) are balanced on a per tire basis. The mechanic puts it in a machine that spins the tire and it tells him where to put the counterweights to make the tire balanced.
Just the one rim is fine. I needed a rim and my mechanic did not mention anything about needing two rims. The general cost for the entire job is $500. You can use your spare tire if you need to save up a few bucks for a new one. My tire went flat. But don't use a donut.
