You use the exactly same amount of double-acting baking powder as you would single-acting baking powder. The difference between the two types of powder is their chemical composition and whether they produce the carbon dioxide gas bubbles that make your baked goods rise when the ingredients are mixed or when the product is heated in the oven. Both types of baking powder produce the same amount of gas, so they are equally effective as leavening agents.
Single-acting baking powder reacts with a water-based ingredient to form bubbles as soon as the ingredients are mixed. If you wait too long to bake your food or mix it too long these bubbles will escape and your food will fall flat.
Double-acting baking powder produces some bubbles when the ingredients are mixed, but most of the rising occurs once heat is applied. This product is more reliable for home baking because it is harder to overbeat the ingredients and the recipe is less susceptible to fail if you forgot to preheat your oven. Because it is practically failsafe, this is the type of baking powder most often found in stores. You'll encounter single-acting baking powder in commercial applications, plus this is the type of powder you would make if you prepare baking powder yourself.