My '06 Jetta doesn't have an AUX input, but will happily read off a data CD with MP3s burned onto it. I can fit as many MP3s on a single as I can squeeze under the 800 MB data cap, so somewhere north of nine or ten albums per disc, depending on how high quality your rips are. Also, remember when you're burning it that the songs will be played in file order, so make sure you have them sorted the way you want to. Numbering the start of the file names (e.g. "01-FirstSong.mp3", "02-SecondSong.mp3", etc.) is an easy way to make sure that happens correctly.
I will echo what others have said about closing out the disc when you burn it; most players don't like open or rewritable discs -- at the very least, it probably won't read anything added after the first session.
Also, Dave H's comment about the discs being less "robust" does ring somewhat true. I find that the last tracks on a disc tend to start skipping after a few months of rattling around in the door pocket. But since it costs a few cents for a blank disc these days, you can just burn a fresh copy when it gets unplayable.