Put a Clamp Integer node down. Plug Current Ammo into the top input. Plug Max Ammo into the bottom input. Plug Set Current Ammo into the output. Replace that Branch with this node.
You are done.
Clamp will keep the value you input between the minimum and the maximum inputs. It will then send that information through the output. The value will never go lower or higher than what you have clamped it to within that node.
What happens to the value after the node is finished is beyond the Clamp’s scope.