Replying to the first post, I somehow have the impression that Xehanort erased the memories of the other five or something. Or maybe he didn't; which was why Vexen, Lexaeus & Zexion actually did plot to overthrow Xemnas in Chain of Memories. (Would you like to spend another few years with the guy who went berserk and stabbed you with a badass Keyblade?)
And of course, with my lack of talent to be a lawyer, I have no evidence to back that up.
Also about the numbers, maybe one of them were like 'Screw this, let's play Rock-Paper-Scissors to see who gets top spot' or Xemnas is just plain lazy and starts pointing out each of them.