It all depends- if you’re a wedding DJ, you might need 4k-5k plus! But in reality, if you’re doing pretty much any other kind of DJing, the right size of your library is the largest number of tracks where you can say with confidence that you know each of them perfectly. For me, that’s right around 900 at the moment, but it’s getting bigger as I practice. Also, the nice thing about the digital aspect of DJing now is that if someone comes up to you with a perfect request and you don’t have it, chances are they either have it on their phone, or you can get it momentarily on your own.