It's not "how much" sand you have, it's how "deep" it is. That's impossible to tell from your post without knowing the dimensions rather than the gallon capacity of your tank. For instance, a tank that is more shallow will have more square inches of floor space which means that more sand will be required to cover it.
A Deep Sand Bed is usually considered a consistent 4-6" throughout the tank. Take a clean dowel, acrylic rod, wooden spoon, handle of a glass cleaning sponge (whatever), and a tape measure. Measure off 6 inches from the tip. Make hash marks every inch from the tip until you have 6 hash marks, then label them; 6 for the furthest away, 5 for the next one closet to the end etc. Then just poke it through the sand and see what depth you have. Don't freak out if it's not perfectly even. That's the simplest way to tell.
Measuring on the outside of the tank is not reliable since most tanks sit a bit below the display cabinet edge. Good luck, my guess is that you have more then enough sand, depending on your rock or other formations.