This game seems to have around ~20 songs in total (unless there's some hidden ones I haven't unlocked) - about half that of the ~40 that come with the Miku games.
Then again, there's some duplication between PD1 and PD2.  And I don't particularly like the vocaloid voices, and K-On offers more "play modes" per song.

It seems that you don't need to get a "perfect" (perfect = combo through entire song) to get S rank.