It's an arbitrary number we picked, based on how long a user might listen to a station. We don't want tracks to begin repeating. Also, a user will tend to listen to the same station for multiple days. This, combined with the fact that they can skip songs (up to a point), 500 seemed like a good number.

We have some stations around 300. Specialty stations have as low as 100. But, those are things like Christmas radio, which people won't listen to too much.

The Film Scores station launched with just over 300 tracks. We're still acquiring the rest. But, my list goes to 514 right now.