My grandfather once asked me: since music is simply the arrangement of notes, and there's (for the most part) a finite number of notes available, how can there possibly be any new music out there at all? Looking at it statistically, everything should have been written already, and every artist is simply recycling ideas. It was an interesting observation, from a man who's main musical interest was opera, though I wasn't prepared to give him an intelligent answer at the time. Now I think I can.
As I'm sure you know, music isn't really just an arrangement of notes. Most of the time, when it's good music, it's multiple arrangements of notes playing simultaneously. That alone increases the number of possibilities from "a whole lot," to infinity. Or think of how much can be done with just one melody. I've heard the Super Mario Bros. theme played by an a capella group, a string orchestra, a ska band, two Tesla coils, a jazz duo, and several other groups. They're playing almost the same arrangement of notes but it sounds completely different each time.
Several years ago I met a guy at a local show in Maine who called himself Neon From Candlelight. He had a single electric guitar and about twenty foot pedals that he would arrange in a half-circle in front of him. He would pluck one or two notes on his guitar and then use the different pedals to warp the sound. It wasn't exactly exciting, but it made a fascinating soundscape if you were willing to get lost in it. Neon From Candlelight shows us that a musician has infinite possibilities with something as humble as a single note.
So I've proven my point to my grandfather. When you combine the potential of multiple simultaneous melodies with the potential of modifying individual notes, you really have no limit to how much you can write. As an exercise, come up with an interesting melody. Now see if you can make a whole suite of songs using just that melody in different styles. Try something slow and something upbeat. Try different time and key signatures (modifying a melody to fit into a different time signature is particularly challenging.) Put different lyrics to the melody. This is something that composers of movie and game soundtracks have to do all the time. It's a great way to both see how notes interact with each other and understand the true potential music has.