How much hydraulic oil does a ford dump truck F - 800 1997 model hold.?

Unfortunately, the dump body, PTO or electrical pump, and cylinders aren't OEM equipment - Ford manufactures the cab and chassis, and an aftermarket provider installs the dump body, PTO, and hydraulic components. Thus, there isn't a standardized dump setup for these vehicles - we'd need to know who the manufacturer of the hydraulic components is.
There'll be a hydraulic tank... where it is can be variable... some suppliers will split a fuel tank and pipe that as a hydraulic tank, some will install a hydraulic tank on the frame on one side of the vehicle, some put it between the frame rails (IMO, the latter option is stupid, as it typically requires you to raise the dump body in order to access the hydraulic oil fill.... kinda difficult to do if you've just repaired a major leak which dumped a bunch of your hydraulic oil out).
That tank might have the capacity listed on it. Other than that, there isn't any way to give you a definite answer without knowing the aftermarket supplier.