The menjo, however, do actually read "11th dan", "12th dan", etc. through 15th. Or at any rate mine do, and I never asked for them to be done that way. Or for the grades or menjo either, the latter just kept showing up in my mailbox 'til I had the whole set. Oh, except 15th, Soke gave me that one during a class at Hombu.
It's different depending on who you talk to, and there are stories about Hatsumi giving different numbers in offhand comments, but it's a sure thing there are 15 ranks. Sometimes people say the last five are simply divisions of 10dan, some say that they are actual separate dan ranks.