How many audio books can 16 GB hold?

It depends on how the audio files are encoded, which determines their size. But, 1gb generally equals about 15 hours of audio. Four gb will accomodate about 60 hours. The Great Gatsby audiobook is about 5 hours long. That means 4 gb will accomodate about 12 books of that length, give or take a few depending on file type.