There is a mechanism to add more courtiers if you have run out but unfortunately a minor bug prevents this from happening. This has been fixed and will be in the first patch. There will also be a new solution for when one of your advisers die. In the mean time you can give up to five kokujin titles to a vassal, and granting him a daimyo title means that he himself can have vassals, which means that you can give him additional provinces that he in turn will hand out to people in his court.