This page:
http://en.wikipedia.org/wiki/USS_Cod_(SS-224)
sayz:
Cod also destroyed another five ships, 24 junks and 11 floating mines as "sunk by gun fire".
Cod's deck guns fired 117 rounds of 4-inch, 50-caliber; 117 rounds of 5-inch, 25-caliber, 1,404 rounds of 40 mm, 200 rounds of 20 mm, and 1,450 rounds of .50-caliber machine gun ammo.
So, apparently, if we discount 40mm ammo and smaller vs ships, that's 234 rounds of deck gun ammo for "5 ships" (no exact tonnage is given) averaging ~45 shells per ship. We might reasonably assume some of those shells were used vs mentioned 24 junks which would put the average number of shells used to sink "5 ships" even lower.