everyone's been wondering how the hell can all of the nort's be together in one time seeing as half of them have been destroyed, and i know most people are thinking that it's probably not really them and they're just a part of the dream. But i thought that this just seems too serious for them to not be real, so I thought that maybe MF from birth by sleep traveled back in time to before any of them had been destroyed and kind of recruited them to help him. Hell maybe they all knew that they were going to be destroyed. Don't know how they would have gotten into the dream world though, them being a dream is probably more likely

It's gotta be in the dream world (since Sora and Riku are younger), so I don't think they're all really there. They're just dreams.

The thing is, people are taking too much on the word Dream in the tittle. It's not really the case.

They aren't actually in a dream, but in sleeping WORLDS. A place separate from the Realms of Light and Darkness.

The most plausible answer would probably be that when a form of something ceases to exist, it becomes a Dream of a world(s) and show up in the Realm of Dreams.

So that'd explain the appearance of the Xehanorts and Vanitas, as well as TWTNW.

About the other 9... One of them could be Vanitas. The others might just be clones or manifestations of Xehanort's Darkness, the source of the Darkness in the Realm of Sleep. Each form maybe created manifestations of Darkness to spread it around the sleeping worlds to get Sora and Riku.