well, time how long it takes to load a song and save out to blank floppy, and multiply by 400?
since the 6k can easily accommodate 20 Technics files to a floppy, that's 20 floppies, but if you save the midis as Standard Midi File you can get maybe 30 or 40 to a floppy so that cuts down the floppies needed somewhat, depends if you've converted any midis to NX in which case save Technics sequencer files instead.