Naomi: Every angel is like a walking, talking battery. (Lights flicker) And that is what happens when we run out of power.

Castiel: I don't understand.

Naomi: Well, then, let me make it divinely simple for you. There are a grand total of nine angels in Heaven, present company included. And one or two on Earth, and... that's all. We are the last men and women and divine waves of light still standing, as it were.