how many audiobooks will fit on an ipod?

Depends on the size of the ipod hard drive, the size/length of the
book, and the quality of the recording (higher quality recordings are
bigger files).  For example, audible.com offers 4 different quality
levels.  For 1 hour of audio the file sizes are as follows:
Level 1 = 2MB
Level 2 = 3.7MB
Level 3 = 7.2MB
Level 4 = 14.4MB

i agree, the audio files vary in quality, so its hard to say how many
fit on the iPod.