Increase Army Unit Numbers?

[Weebeast]

Occasionally I'll see an AI stack with 21 units in it, I have no idea how or why they do it though.

You miscounted. The army window in city and the one outside are laid out differently making you think there's more unit in that stack.

I'm seeing if I could do that for the AI, so the AI fields like 30-40 some units while I stick with the regular 20 unit stack...hopefully that'll be more of a challenge.

Just wait for the reinforcement to meet the main army. Surely they can control more than 20 just not spawn them all in the middle of field.

I do this all the time except late in the campaign when there's a big chance of after-battle ctd to happen. Use toggle_fow, calculate enemy movement and strength, adjust your stack accordingly.