I use this method of ammo supply in my games. I give each side a number of blank business cards with the artillery ammo type printed on the cards. When they call a mission, they give up a card regardless of who calls it. I find it works rather well especially in multiplayer games where they have to share the pool. It’s simple, quick and doesn’t have the errors I’ve seen in more detailed supply rules. The benefits outweigh the abstraction for me.