For every one legal mp3 download (and there's lots out there for free), there are, according to this estimate, 16 illegal ones. That seems like a small number, but the absolute values are staggering:

The Yankee Group's Michael Goodman puts the number of 'legal' DRM-encumbered downloads at 330 million last year - compared to 5 billion downloads from the P2P networks. So for every 'legal' purchase, there are 16 illegal downloads.