What I have been told is, that 8. dan is the highest rank achievable as long as you are looking at ability within the art. 9th and 10th dan however are grades handed out very rarely as signs of gratitude for an extraordinary effort to develop and/or spread Aikido throughout the world - usaully as a life-long commitment. Actually I think it is possible to 'jump' grades from say seventh dan to 9th. dan if a person for example travels to a new country to try and start up Aikido there, but I could be wrong.
In most Aikido organizations, the highest tested rank is around 4th or 5th Dan. After that, the ranks are given out according to how the Powers-That-Be
think you've contributed to the art or their organization.
Almost all rank promotion above 5th Dan are totally political in nature. That's why Aikikai only has living 9th Dans. Its hard to get a bunch of 9th Dans together and decide who is going to be a 10th Dan. If you look at Judo's history, their last 10th Dan was Mifune Sensei, who was given his belt by Kano, Judo's founder.
A beginning student might look at two dojos and decide to attend the school who's head instructor is - say 6th dan because the other school's instructor is 3rd dan. However that could be a quick, shallow judgement of the situation.
That 6th dan might have other schools to take care of, so you'll really be instructed by some shodan or brown belt. That 6th dan could be able to do
aikido perfectly, but be unable to transmit that knowledge because he/she is the world's worst teacher!
There is no quick and easy way to make judgements about people. You certainly can't sum up a person's life by a belt. Belt number worship is almost as dangerous as the belief that you
need a higher belt, to prove you know aikido.
I thought I'd throw this stuff in, because it is all implied when you discuss high-ranking martial artists.
Rank inflation is not as bad in aikido than it is in some other martial arts. In some martial arts, you can only consider belt ranking as a marketing device.