I haven't heard a final verdict on how many tracks you can record simultaneously in GarageBand.
I *believe* the maximum is a single stereo track.
This presents an obstacle for recording drums, as you'd need to mix them down to stereo (using an external mixer) to record them, and you can't adjust individual mic levels on that submix afterwards (obviously).
You're also stuck with multi-session recording (recording each instrument in a separate take, one after the other, as opposed to just recording the basic tracks together, in a single take), which can be a problem if you're not very experienced in studio recording. Things can tend to get a little untight.
-s*