Oh, well, Sora killed his heartless, and Sora and Riku killed his nobody...which might have been part of the plan, come to think of it. Sora also killed two of his vessels. ...and I never saw the old guy version of him before, so he might have got dead sometime. Besides, I mentioned the plan includes time travel, remember?

As far as a thirteenth him...I'm pretty sure he'll try for Sora again, because Sora's kind of special. It doesn't explain why anyone else is here, though.