What is the average VOLUME of dump truck?

That depends on what type of dump truck, what type of commodity, how it is measured (by weight or by cubic yard), and the state laws regarding weight allowances.

A Class 7 single axle dump truck can typically haul eight to ten cubic yards, or up to eight tons of payload.

A tandem axle dump truck can typically haul 15 to 20 yards, and anywhere between 12 and 16 tons of payload, dependent on truck specifications and local laws.