Tom -
I am unable to offer a good source of the historical data you requested; but I would like to suggest a few reasons why the actual number of shells fired to sink an individual target may vary widely.

Underwater hull damage is most effective in sinking a ship. Guns of the sizes carried by U-boats generally cause a comparatively small diameter hole in the hull; and the damage will not contribute to immediate sinking so long as it remains above the water line. Waterline hits were difficult to achieve even at close range, because a low shot will often ricochet off the water surface. If the target had several watertight compartments, it was usually necessary to damage more than one to cause sinking -- particularly if the ship was not fully loaded. Tankers, with multiple watertight compartments, sometimes containing oils with density less than water, were notoriously difficult to sink.

Even after a target has been damaged sufficiently to sink, the rate of sinking might not have been rapid enough to be detected by the U-boat; and firing might have continued. The rate at which water enters a ship also depends upon damage control measures taken by the crew. Holes can be patched and a certain rate of flooding can be pumped overboard -- as long as the target has power and sufficient pumping capacity.

U-boat guns were more effective at starting fires. Small wooden ships were particularly vulnerable; but fuel, flammable cargo, or superstructure materials (including paint) of larger steel ships might be ignited by the projectile bursting charge or by hot shell fragments. Firefighting efforts by the crew might cause flooding of watertight compartments, and smoke and heat often caused the crew to abandon stations necessary to maintain damage control efforts.

Comparison to warship damage by shelling is not particularly helpful, since warships typically had more watertight subdivision per unit volume or weight. In the case of (surface) warships there is truth to the old saying that enemy shells don't sink a warship -- warships are sunk by their own ammunition and fuel after the enemy shells provide an ignition source. =AL=