Well, I'd say definitively between 500 and 1000. My Youtube has 850+ favorites on it, the vast majority being music, and I have found a ton more music that I can add; it has the potential to expand beyond that in less than a month. I limit myself to about 500 songs on my MP3 (seeing as it's only a 4 gig). But some songs are only on my MP3, while still others are only on Youtube.
Either way, I have a mid-sized collection in contrast to some kids, but in comparison to my little brother and some of my friends, my MP3 is a wonder. I have had my Youtube favorites called the ultimate playlist of Rock and Techno by a few people; I have artists like Renard, Mayhem, DJ Splash, Amorphis, Disturbed, Marilyn Manson, The Prodigy, System of a Down and even Cascada. I never get bored of the music I have, and I avoid specific loops with by just closing my eyes and choosing a song to listen to.
So, my music collection is not only rather expansive, it's also diverse despite being defined easily by two main genres.