Already mentioned a lot but just to toss my 2 cents to the devs.
Why are you even considering this feature? Everything about your game indicates that you are testing a player's knowledge and understanding of ACW army level tactics ie cover, use of artillery, when to concentrate troops and when to keep them dispersed, timing of attacks and not unduly committing units in combat to keep them fresh.
Ammo does nothing except test a player's ability to be a bean counter. If this was Ultimate Quartermaster Gettysburg, I would understand. The player, in the game, occupies the position of someone like a Longstreet or AP Hill on the map while the branching scenario options has the player occupying the role of a General Lee or Meade with respect to overall Army strategy. These people did not typically concern himself with a brigade's ammo supply. He drew up plans, issued orders and if a unit commander told him he couldn't go for whatever reason, the orders changed or he was told to make do.
Your game actually does this very well, the AI's shortcomings notwithstanding. Don't ruin a good game with 'feature creep' like ammo just because its 'more realistic' in someone's eyes. Adding ammo would actually make it less realistic as the people who's position you occupied didn't worry about such things, they had subordinates to do that.