If you consider how many songs you have that are REALLY REQUESTED, it will probably be under 1,000... IMHO.
Generally speaking, I would estimate a good "party" karaoke setup has 250 discs (to get started mind you), which on average of 12 songs/disc is 3,000 songs. A good professional will have around 6,000 songs to start with.
Consider who you play for. If they are the young and restless, you need the pop songs coming out now. If your audience/singers are folks over 50, then the oldie goldies are what you want... 60's and 70's typically.
It is a rare Host that needs tens of thousands of discs... again , IMHO. I expect some of our users would take issue with me.
Shop wisely for your audience/singers. Write down what they request if you don't have it, and go buy or make it.
Another way to round out your library of songs is to create your own from music. Our Vogone does the best possible job (not total removal mind you) of vocal removal, and our Karaoke Home Producer allows taking your music and the lyrics text and sync'ing the lyrics to the music just by playing the song and tapping the space bar where each word should start. It truly is very easy. Many Hosts use KHPro to redo songs they bought where the "commercially placed" lyrics are out of sync. Sweet Georgia Brown is notoriously bad for this.