17 for Reel Big Fish to Arctic Monkeys ( http://labs.echonest.com/SixDegrees/?start=reel+big+fish&... ) - that's the highest I've got so far.
EDIT: Oooh, 18: The Black Dahlia Murder to The Zombies: http://labs.echonest.com/SixDegrees/?start=the+black+dahlia+...