Let me first of all clarify you that there are different valves used in a diesel engines. Now it depends on which valve numbers you want to know. The different types of valves are listed below:
Inlet valves - inlet valves generally allow the intake air to enter into the combustion chamber. These may generally vary from 1 to 2 per cylinder in numbers depending on air quantity and open timings.
Exhaust valves - likewise they allow the exhaust gas to allow burnt out gas from chamber to exhaust manifold. They are also 1 to 2 per cylinder in numbers.
Cylinder block pressure regulating valve - pressure regulatimg valve is used to regulate the amount and pressure of lub oil entering into the crankcase from attached lub oil pump.
Pressure regulating valves for cylinder head - cylinder head also require lub oil and that flow is controlled by valves adjusted by screw/knob. These are 1 in number per cylinder.
Air distributing valve - commonly known as distributor also acts as an valve by sending the starting air to the cylinder as per firing order or to the cylinder which is in firing position. 1 in number per engine.
Starting air valves - they are attached to each cylinder head and operated at starting of engine. It allows the starting air to put pressure on the piston to initialise cranking. 1 in number per Cylinder.
Air check valve - as the name suggests, it checks the air and stop it from entering distributor without the manual consent of operator. 1 in number per engine.
Pressure relief valve - It is a safety device which is mounted on each cylinder head which releases the air when pressure is beyond the limit. 1 in number per cylinder head. Sometimes crankcases also possess relief valves.
Solenoid valves - For electrical operation of engines, a solenoid valve can also be used per engine.
Fuel injection valve - Also known as Injector. Yes you read it right. Injector is also a valve which allows the fuel to be injected in the combustion chamber at certain minimum pressure below which the valve won’t open and restrict fuel flow. 1 per cylinder