There are a few sources.
There’s this thing from the Broker place:
Six realms. Many intersections. Countless fractals.
In balance? Possibly. But that might be wrong. If six equal one, then what is the other? The one outside the pattern.
And then there’s this item from Korthia:
The seventh covets what the six hold fast. The fulcrum wavers. All will be undone.
And then there’s that weird Broker guy from Zereth Mortis, Firim:
The six forces that pointed toward a seventh, and yet denied it. For a long while I considered this seeming contradiction no more than another variable. An unknown waiting to be solved.
But the song of the oracle continued to reverberate in my consciousness. And as I allowed my focus to soften, my grip upon the tangible to loosen, the geometry took shape in my mind.
It was both six AND seven. The six were one, and the seventh the other.
Did they desire union? The song seemed to say otherwise. Both were, yet only one could be.