From Kvasir the Younger's "Looking Glass of History"

...Why the Nine? To this day, after all, there are far more than nine Dark Lords. While many state that the Nine are the most prominent of Dark Lords, one must question this--is the are the Sekhmetides of the Blasted Heath truly more powerful and influential than the Mongranes of the Crossing? And while the Great Khans of the Howling Waste were a major force, the present Dark Lords of the Waste are nothing more than a title held by the Cthonique heir. Are they truly more worthy of membership in the Nine than the Belfiors?

And again, why Nine? Why not Ten, or Twelve, or Six? The Pallasians say that it is symbolic of the Nine Created Beings of Mother Night--the Dragon, the Light, the Sun, the Moon, the Earth, the Water, the Wind, the Air, and the Fire. But the Pallasians have ever been few in the Lands of Night--could they have influenced such an essential thing? We turn to the past, but the way is closed to us--all our earliest references are to the Nine, and they speak as if talking about a long-held custom.

In the end we are forced to conclude, the Nine are the Nine, because they are the Nine...