Does 12 have a symbolic meaning in Japan?Imo, they awarded that 12th dan so that noone would ever equal him in rank.
If he'd 'only' be 10th, then there would be people at the same level as him.
I guess that is why they made it a 12th instead of an 11th. That way they retained the option of someday instituting an 11th dan without having the same problem again.
Of course the kokodan can do whatever they want. They could award an 11th if they wanted to but at this time, that rank does not even exist.
This is not different from the 9th and 10th dan kendo. They still exist, but they are never aarded anymore.