The stars are cumulatives! The game have 30 songs, which song can give you 3 stars, but you need at least 12 stars for Starting Steps, 18 for Smooth Moves, 24 for Fancy Footwork and 8 for Workout mode. At least 63 stars, because the Fancy Footwork need 24 stars to unlock the last song. If you get 3 stars on every song, including Workout mode, you get a total of 93 stars.
Since you need 5 hours of solid dance, finish your game and start a song over and over letting it play by itself. The game give you one star per song.