Many people have pointed out that we seem to be at more than thirteen seekers of darkness now.  If that is true, why is that?  At the end of Dream Drop Distance, it seemed like all but one had been decided.  However, now there seems to be more than Xehanort needs.  Well, we do know that Xehanort has a history of having backup plans and backup plans for his backup plans.  So, it wouldn't be too crazy to assume that he would have backup seekers of darkness.  A line that Xehanort says to Terra in the secret ending to Birth by Sleep could confirm this.  After Terra tells Xehanort that he has a plan to free himself from Xehanort's control, Xehanort says to him, "You are just one of many roads that I might choose to take.  Trust me.  I made certain of that."  What do you guys think?

Many people have pointed out that we seem to be at more than thirteen seekers of darkness now.  If that is true, why is that?  At the end of Dream Drop Distance, it seemed like all but one had been decided.  However, now there seems to be more than Xehanort needs.  Well, we do know that Xehanort has a history of having backup plans and backup plans for his backup plans.  So, it wouldn't be too crazy to assume that he would have backup seekers of darkness.  A line that Xehanort says to Terra in the secret ending to Birth by Sleep could confirm this.  After Terra tells Xehanort that he has a plan to free himself from Xehanort's control, Xehanort says to him, "You are just one of many roads that I might choose to take.  Trust me.  I made certain of that."  What do you guys think?

I think you are onto something.  I also think that his nobody Xemnas also recruited many "special" nobodies other than the main thirteen we know of.  Axel did mention "Turn me into a Dusk".  The first Organization was Xehanort's first attempt at the true Organization and he had nine years between after the climax of BBS and KH1. 

I think as of right now he has most, if not all of them. As for backups, yeah I believe Aqua was meant to be one of them.

I mean, Xehanort had like over a decade to gather vessels.  I guess, the question we should be asking is.  What does he do with the "useless" vessels?