Yes you are right that putting reinforcements under AI command gives you more troops overall. I had completely forgotten about that. Though the reinforcements AI is suicidal all the time (at least that's my experience).
As on the fullstack it's definitely 20 units. Look at this army of jerjes78: It's just the fact when you open the enemy army composition scroll it displays 2 lines x 9 unit slots, so it seems that 18 is the fullstack, but when AI has 20 units you can see them on the third line.
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