Lava rock can be used as decor or as extra media in your canister filters. Lave rock is very pourous which provides a large amount of surface area for the growth of beneficial bacteria for your tank. 
Each bag of lava rock contains approximately 8 oz of rock. Please keep in mind that each rock will be different in size, shape and red tones. Some rocks may be a brighter or darker red than others. Each bag of rocks also vary in quantity because of size and weight. You are getting 8 oz of rock and not a specific quantity, shape, or size.