Arrr... So how do you generate MUN's (munitions?) And how are they expended?
I take it you are referring to an overall stockpile of MUN's that anyone can tap into provided they are on the supply grid?
Ammunition is for artillery units. Normal units are either supplied or not, ammunition spending of them is not tracked.
Each round of combat where you use an artillery unit as support costs one MUN.
You get MUN by producing it, setting a turnly flow of it during the quarterly Interphases. Obviously, more munition costs more, plus, the max you can get is limited by your level of war industry. (by political decisions, you can gradually switch your economy for war production. But beware: the more you concentrate on war, the less consumer products you will have, lowering your national will)