Ok. We know this: Xehanort, Young Xehanort, "Ansem", Xemnas, Braig, Isa.
6 are left (to complete the 12 seekers that he got atm, he still need 1 more.)
He like "say" Terra is one , so that means 5 are left to guess.
As we know xehanort has the keyblade of the 6th foreteller. Young Xehanort didnt have that keyblade till later on. What if, the 6th foreteller told xehanort a way to re-made the Keyblade war, and told him to allow his heart inside him?
If that happen, that will explain the Dream we see in KH X, in wich a person in org.Coat talks to the other *5* foretellers. Since Xehanort has the 6th foreteller heart inside of him, he will be that person too : Xehanort + 6th foreteller, that way it was possible for him to travel back in the time (the self that will be waiting in that destination will be the 6th foreteller) before the keyblade war and convince the 5 foretellers with tomes of prophecys to be the other 5 darkness and accept xehanort heart.
https://www.youtube.com/watch?v=hlL2VAiYWI0