To find the correct amount, you must consult your truck's owner's manual. The information should be in the section dealing with the air conditioning system. If you don't have the manual, you can often find a digital copy online by searching for "[Your Specific Chevy Truck Model Year] owner's manual" (e.g., "1994 Chevy C1500 owner's manual").
Alternatively, a local auto parts store may be able to look up the specifications for your truck using the Vehicle Identification Number (VIN). However, always double check this information with a reliable source like your owner's manual. Improperly charging your AC system can damage it.