One would have to go through all the Fox-Sterlingworth production cards to get the number. I doubt Cody will ever do that.
From my 35 years of recording observed guns in the 12-gauges they seem to have been built in blocks. I've recorded a couple in the 1306xx serial number range with long backstrap trigger guards which were probably proof of concept guns. Then I've recorded four 28-inch barrel guns from 135605 to 135793 but I've recorded regular Fox-Sterlingworths in that range as well. The great bulk of the 12-gauge Fox-Sterlingworth Skeet & Upland Game Guns are in the 160xxx range. I've recorded 21 from 160076 to 160434. These guns are a mix of barrels marked
STERLINGWORTH FLUID COMPRESSED STEEL and
SPECIAL ALLOY -- FORGED STEEL. Then a run of regular 26-inch barrel Fox-Sterlingworths. Finally, six more Skeet & Upland Game Guns from 160722 to 150818 all with
SPECIAL ALLOY -- FORGED STEEL barrels.
How many more guns in these ranges were made is impossible to say.
When introduced in Savage's 1935 A.H. Fox catalog only 26-inch barrels were offered.
Fox-Sterlingworth Skeet & Upland Game Gun 1935.jpg
By the 1937 Catalog they also offered 28-inch barrels.
Fox-Sterlingworth Skeet & Upland Game Gun 1937.jpg
The Fox-Sterlingworth Skeet & Upland Game Gun last appeared in the 1941 catalog --
Fox-Sterlingworth Skeet & Upland Game Gun 1941.jpg
A magazine ad from 1937 --
1937 Racy Skeet Model.jpg