There are about 37 hidden songs.
The easiest and quickest way to get this is by playing Quest Mode, which will help you in the long run as you complete Clubs and work on Universe Tower achievements. Every Club Challenge has a song that you can unlock, if you knock out your opponent. There are also songs that you unlock while playing Universe Tower, so as long as you beat Quest Mode and knock out every opponent(which is simple enough) that you come up against in challenges, then you should unlock this after All of the Clubs and Universe Tower. Basically beat Quest Mode and you'll have it.
Alternatively you can unlock them all by clearing so many song in Game Mode, but it will take longer, as you will have to go through Universe Tower and the Clubs anyway, if you are trying to unlock those achievements. Plus it's easier to knock out opponents than unlocking songs through Game Mode.