It depends on whether you're going for a planted tank or not. Planted tanks require a minimum of 2&1/2-3 inches for easy planting and the roots to take. That's roughly 2 lbs per gallon for a regular tank.
For a non-planted tank, gravel is optional so whatever floats your boat. 5lbs will barely cover the bottom of a 10 gallon tank, you may have difficulty arranging decor that needs to be partially buried like fake plants.