Each unit of retinues starts at a very small number and reinforces over the next several months to its maximum.
For example, in my current Venice ironman, I have my entire retinue cap in a doomstack of pikemen (wars go by faster if you have the most efficient soldier at assaulting holdings and when a well-led unit is fighting a significantly smaller unit, it really doesn't matter what the army composition is). Each unit started at 1 and reinforced up to 400.