I know they get auto generated if you don't have enough for your major families but I don't know if it takes into account open jobs.my understanding used to be that minor characters get generated automatically over the course of the game, so you always have people around to adopt and give offices to. Lately I'm not sure of that anymore, I've reached the point in my Rome campaign were I don't have enough characters to serve as commanders for all my legions, offices, governors and researchers.