Suppose all 13 organization members were all worthy of being in the organization, what would happen to Ansem (SoD) if they had been? The organization can't have more than 13 seekers of darkness, so what would happen?

 

Oh wait 15 members!! Forgot to count the old coot.

Then Ansem and Xemnas wouldn't be needed I guess.

I'm pretty sure the other organization members would be picked off before Ansem SoD...unless it would just be easier to use these members...they would be able to ignore the time traveling plan and just move forward as is. A lot less interesting.

Well they can have more, they just need a minimum of thirteen because that's how many they need to recreate the XBlade. If they were all worthy they would have all been seeded and there would be extra seekers.

Ansem and Xemnas would merge and become Xehanort after gathering the 7 princesses and the 13 Darknesses.