I think the Sovereign [Skill/Stat or Special Ability] should allow how many Champions you can have. Also larger Armys should require a Champion or a Created Command Unit.
Sovereigns being your Flagship should always lead the largest Armys. Champions should have a skill/stat that allows them to also lead an Army. Created Captains, Commanders, Generals, ect should also have a "Pack" to Lead an Army. I think the most you should be able to send out is a Company or 20 units without a Captain. To get into more depth and complexity maybe higher tier or expensive units need more command points to lead.
Example:
Your Captain has 101 Command Points
Peasant = Require 1 Command Point so he could Lead 100 Peasants
Knight = Require 2 Command Points so he could lead 50 Knights
Custom Super Unit = Require 3 Command Points so he could lead 33 Super Units
or a combo
50 peasants 20 knights and 3 Super Units
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Champions should be harder to get, and all armys need a leader. Also Captains could level from titles "Captain +XP--> Commander +XP--> General."
Sovereigns Could start out with an ability such as "Faction Leader" which basically would be an amplified Army Command Points so he always has the largest army. Or just put a cap on Champions/Captains