The most popular contemporary music stations have playlists of about 150 - 225 songs, classic oldies/rock stations such as WMGK-FM Philly about 400 songs and Hot Rock stations like WMMR, PHILLY & KEGL-FM DALLAS, 300 - 400 songs.
Contrary to popular belief, a bigger playlist doesn't mean more listeners. I learned this the hard way. Our playlist was 1,200 - 1,400 songs and much to my dismay, we had very few listeners. Disheartened and humbled we finally met with a consultant & also became subscribers to Mediabase. Everything I thought I knew about radio and playlists I threw out the window. Everything! With new found knowledge in-hand, we completely revised our playlist and now have more listeners than ever before. Our playlist hovers between 200 - 220 songs.
The key is this. Every time someone turns on your station the song they hear has to be the best of the best. You're not going to hit a homerun all the time but you try.