There are currently 10 soundtracks. 6 movie soundtracks (movie 1 has no specific soundtrack) and 4 series soundtracks. The other cds are songs only.
*The three first series soundtracks are used all the time in the series, from the very beginning
*The fourth series soundtrack has specific Drum and Alabasta tracks that are still heard in the later arcs.
*Movie 2 soundtrack was used in the series from Whiskey Peak onwards
*Movie 3 soundtrack was used in the series from the Alabasta battle onwards
*Movie 4 soundtrack was used in the series from Skypiea onwards
*Not sure if movie 5 soundtrack started in Skypiea. It is used in the G8 arc onwards though
*Movie 6 soundtrack was used in the series from Water Seven onwards
*Movie 7 soundtrack just started being used in the series in Enies Lobby