Ground Commander

Hey, so I'm making a medieval era map and I ran into an issue. I want to make ground commanders (Kings/Sultans), however, I don't want to make it so that you can make more than one during the reinforcement turn for obvious reasons.
How do I make it so that the ground commander per scenario kingdom's capital is a one time thing?

When editing the unit un check the box that makes it buildable, then add a single one to the factions as an event on the first turn

Thank you