There is discussion about this in another thread. Thought I would post what I know.
1) 20 is the maximum number of units you can control in a battle. Thats one full stack.
2) If there are more than this then you have the choice of:
a) letting the AI control other stacks or
b) you control them as they come on as replacements for lost units. You dont get to control which order they arrive.
3) The choice is via check box(es) on the battle deployment screen.
4) I read that the patch you are on makes a difference. Dont recall all the ins and outs but V1.5 works well.
5) I read that the game can ignore your selection in the check box. To get around this multiple check/unchecking of the box works. I do this and dont have any problems...whether its this or patch (or both) Im not sure.
Hope thats clear and accurate.
Brutii: http://rtw.heavengames.com/cgi-bin/forums/display.cgi?action=ct&f=10,2359,,10
Gaul: http://rtw.heavengames.com/cgi-bin/forums/display.cgi?action=ct&f=10,2381,30,100
German defensive: http://rtw.heavengames.com/cgi-bin/forums/display.cgi?action=ct&f=10,2435,0,20
Huns : http://rtw.heavengames.com/cgi-bin/forums/display.cgi?action=ct&f=10,2507,,20
Sassanids (BI): http://rtw.heavengames.com/cgi-bin/forums/display.cgi?action=ct&f=10,2521,0,45
Celts & WRE: See War Stories thread