I'm guessing that each level has its own, separate set of songs. So games with more stages will have more songs, but: a) the average number of songs per level will be fairly standard and b) the songs will not be interchangeable from level to level. That's just my hypothesis. We've only seen one stage per series at the moment, so I'm having trouble visualizing an example... but I think if there are 2 Mario stages, Delfino plaza will have maybe 7 songs (the ones in the screenshot and then some), and "Peach's Castle 3" (a level I just made up) will probably have 5 or 6 songs, like a remix of "Peach's Castle Theme" from Mario 64 or Paper Mario, and maybe the main theme from Super Princess Peach, etc. I don't think there'd be any overlap in the song choices between two Mario levels, either. My guess is that each level will have songs chosen and remixed to match that level, not just any song from the franchise.
We'll see if I'm right, though.