You can't have more than around 240 men per unit. If you want that many, you have to 1) set your unit size to Huge under video options, and 2) edit data\export_descr_unit.txt so that the unit in question has a size of 60 (this number is multiplied by four for the Huge setting). This may unbalance the unit, but that may not be a problem for video purposes.

Also, always remember that you can have multiple armies of one faction in a battle.

-Simetrical