How to control more than 20 units armies ???

[tono4850]

So my reinforcements can enter a battle without having to wait until some of my units die completely (It´s the worst in siege battles where battles are fought to the last man standing)

Is it possible or hard-coded ???

[Boriak]

I've never heard it done. Either you assign the AI to control the second army or you have to wait until your units either die or retreat them.

[Goffredo85]

It's very simple indeed...just go play Rome 2!!!
Jokes apart the rome 1 engine does not support such a feature. The first game in which it was possible to play with 40 units army was empire:tw, and it was unlocked only through modding. The first title to officially offer large battles was shogun2 fall of the samurai iirc.