Re: "super" high ranking yudansha?
As of last year, the certificate number for a 6th dan (Aikikai) was in the mid 600s. Which means that there have been a grand total of 600 something people who've reached that rank. This includes those who've gone on to higher ranks, but I'm pretty sure doesn't include the people who were given 7th and above without going through the 'lower' ranks.
Not what you were asking for exactly, but it's interesting.