8th, sometimes 9th or 10th...
In Aikikai the highest grade is 8th dan, according to the rules - but those rules also say that still, in some special cases, 9th and 10th dan can be awarded...
Again in Aikikai, every dan grade above 4 dan is given by recommendation, involving no testing. There are minimum requirements of years of training between the grades:
5 years from 4 dan to 5 dan
6 years from 5 dan to 6 dan
12 years from 6 dan to 7 dan
15 years from 7 dan to 8 dan.
To my knowledge there are no minimum requirements for 9 and 10 dan.
Aikikai Hombu dojo decides about all recommendations.