I rip my songs at 192kbps which is good quality (128kbps seems to be more common but I can really hear the degradation in quality at 128k). That makes a song about 7-8 megabytes, so at 700 megabytes for a typical CD, that implies about 100-120 songs. Still not bad, though.
Lower quality may be fine in a car, though. The phrase my audiophile brother-in-law uses is "noise floor". He means that cars are noisy and it's hard to hear the distortion in the music when you can hear the engine and other traffic and the noise of your tires on the pavement and the noise of air resistance from your car. If your car deck will read CD-RWs, you can try different bitrates without wasting a CD (you can erase CD-RWs and reuse them) and find out what's acceptable to you.