It's really very simple:
1. Your trade page will tell you how much of a particular item you use each month. as far as ammunition, fuel and other consumables that varies with the number of active units.
2. When you set up a military unit of any kind, not only do you have to have the minimum defaults of those particular items but strategic stock remaining for the supply units to draw from.
3. You have to have supply units to restock whatever the equipment uses in a month. If you do not have enough strategic stock to replace whatever is used the unit will dismantle and the equipment moved to deactivated.
4. If you do not have enough supply units to get to all your military units eventually, they will disband due to lack of supplies no matter what your strategic stock levels are. How long it takes in this case I have no idea but if it's taking longer than a game day to replenish all your units at least once (esp. during a war), you need more supply units.
I've seen it many times (and even did it myself when I first started this game), players will load the maximum amount of ammunition into their units on set up and not have enough strategic reserve. Look at it this way: If the supply unit goes to the warehouse to get stock and there is none or even not enough there to bring a unit back to 100% supply something is going to get disbanded.
In addition to the above reason there is another major reason NOT to do this. If a unit is destroyed in battle, unless they've changed the game dynamics all that ammo along with the other supplies goes with it.
Every unit in the game documentation tells you how much ammo is consumed per round. The question is, how many rounds do you want it to be able to go. I personally set mine up for 10 rounds which is WAY too optimistic. A player that knows what he's doing, and equipped right will kill any unit in one or two.
A far as strategic stock on ammo, fuel etc., I'm still working on it as I've just came back to the game, but I won't start feeling comfortable until I'm around 10X the total in all my military units. That's just the starting point, my strategic stocks will always be increasing.
A big part of this game is logistics. You can't fight without beans and bullets.