I just declared myself as emperor (Yuan Shao, 190 start) and maxed out the prestige for assignments (8). Additionally I have 7 commanders with the nobility skill in court positions that should provide me with more assignments, for a total of 15. However, the UI still only shows 10 assignment slots. Is this a bug? I can provide a save if it helps. The other faction-wide bonuses, corruption, satisfaction, etc... all seem to be working.

Edit: Apparently this has been a bug in previous versions of the game also:
https://www.reddit.com/r/totalwar/comments/cj1quo/maximum_number_of_assignments/
https://forums.totalwar.com/discussion/245408/assignment-limitation
So I'm guessing that there is a cap that is misconfigured somewhere.