I don't no if there is a limit on the amount of different units that can be made but if there is I have an idea that might help with the problem.
I'm willing to bet that I'm not the only one to have noticed that there are a lot of units in this game that are almost exactly the same that it doesn't make since to have them both. Example the Byzantine dismounted lancer and the Byzantine swordmen infantry or spear militia/ town militia and numerous other units.
So I was thinking to free up some slots for different units the simple solution is just to eleminate the duplicate units. What do you think?