I beleive that this is already programmed in the game as can be seen in these photos. It just needs utilization.
I suggest that this raise banner action should be automatically enabled once there is a group of 30units of the same type.
This will help the players to easily assess how many units he needs to fight his opponent’s army. It’s like a bench mark, a symbol that will tell me that “oh he already have 30units of archers, i need to produce 30 horsemans”.
To the opponents perspective. That will serve as a way to taunt the enemy.