An update has been released the Workshop in Zombie Slayer today which auto populates the "# to craft" dropdown with the maximum allowable based on your inventory. To take use this choose which item you wish to craft from the Blueprints list and click the "Autofill Items" button. This will populate the workbench and you will now see "Max (#)" as the default selection in the "# to craft" dropdown. Note: the "Max" option will be removed as soon as you make any changes to the workbench items. This is only applicable when using "Autofill Items"