Some items in the game natively won't show stack size count.
Example, weapons will only show the bullet count that is loaded in the first weapon which means you can stack the guns but you will not be able to see the stack counts.
This is something facepunch did & it's locked client side, theirs nothing i can do to change that.
Unless i make a custom UI overlay for every item inside every container that exists in the game which would lag the shit out of peoples client etc.