Three - the two drive axles, and the steer axle.
The average tandem axle truck can carry about 12 cubic yards of soil. Generally, a tandem axle truck has three axles.
Five, all told. The steer axle isn't included in the count, so a single axle with have two - a steer and a single drive axle, a tandem will have a steer axle and two drive axles, a tri-axle will be a tandem with an additional lift axle, a quad axle will have two additional lift axles, a quint will have three additional lift axles, and a centipede will have four additional lift axles. A superdump is usually a quint with an additional Strong Arm mounts to the top of the dump body which extends out to maximize the vehicle's wheelbase.
2 you fool
It can have anywhere from four (a small pickup with a dump body) to 22 or more (tractor and end dump trailer with lifting tag axles). For a straight truck in the US, the most wheels you'll get on a vehicle is a configuration which is known as a 'centipede' - it's a straight dump truck (10 wheels) with four lifting tag axles (8 more wheels), for a total of 18 wheels.
Provided it's not a short tandem, 15 yards with a 'rock tub' dump body and 16 yards with a 'dirt tub' dump body are typical.
They come in many sizes and configurations. They could be single axle trucks, tandem axle trucks, tandem axle truck with additional lift axles, tandem axle trucks with a twin steer setup (and these may also have additional lift axles, as well), and even tractor-trailers with mixer trailers. You'd have to be a little more specific as to which one you had in mind.
Dump trucks come in many shapes and sizes, and the same holds true with dump bodies. For a tandem, 13 - 15 tons is typical.
Dump trucks typically don't haul liquid commodities, unless you're thinking of snow removal operations where they'll be outfitted with prewet or calcium chloride tanks. Even with that in mind, you need to remember that there are many different sizes of dump truck out there. 15 cubic yards is "typical" for a tandem axle dump truck.
I would like to see 20 cubic yards on a tandem. I would hate to pay that overload ticket.
Typically, and legally, around 15 tons, give or take depending on the tare weight of the vehicle.