Providing a Max Quantity in the Price List will help you control how many units of a service your Techs and your customers can choose.
To add a Max Quantity first navigate to Configuration->Price List.
Once you are in the Price List locate the item you want to adjust click the and click the stacked dot menu. Doing so will bring you to the Add Service menu.
Once in the Add Service Menu, turn on the Set Max Quantity switch and input your max amount into the Max Quantity field. After you input the information into the correct field click Save.
After saving, new jobs will show a pop up whenever too many of a Service are chosen and instant booking will also limit the Service.