All of them. Audio CDs subdivide tracks with index marks. They are rarely used for general purpose and few hardware or software players expose index navigation now. However, every track has a lead-in at index 0 and the main program at index 1. When you skip tracks you start playback at N.1. You only hear N.0 when playing through from the previous track. There is no limit to what can be in the lead-in and some discs would hide bonus "tracks" in 1.0 which is often skipped over by hardware players but can be manually navigated to with the index buttons.