If it's a positive displacement pump, your best way to regulate/control flow is by controlling the pump's speed or stroke using a flowmeter or by feedback from a tank weighscale.
If you don't want to do that for some reason, you can use the pump as a mere pressure source by putting a back-pressure regulator on a tee in the discharge line, directed back to the tank, sized for the full output of the pump (note- not the suction of the pump- that may cause overheating due to recirculation). Then you size a control valve/flowmeter combination for the forwarding line as usual.