Add required condition in decision box, If exceeds then throw validation feedback activity
The UI or the Button where the User is applying leave should be converted to a Microflow.
In this MF, first check if for this month did the user cross the ALLOWED leaves or not. If no, let him continue. If yes then dont let him continue and show a nice message box.
Also on the contrary you can let him continue but then make sure the validation is there on APPROVAL level.
Hope this helps