To find the exact refrigerant capacity, you must consult your truck's owner's manual. The information will be in the section on the air conditioning system. Alternatively, you can find a sticker under the hood that may list this information. If you don't have the manual, you can usually find a PDF version online through a Chevy parts website by entering your VIN.
Attempting to add refrigerant without knowing the correct amount is risky and could damage the system.