Right. In that case, I'd like to suggest the addition of an indicator in the sub construction window, or possibly next to the dock size indicator. Or both.
My guess would be that the number is not only dependent on the fleet size but also the country you play. As I said, this is only a guess, but it seams reasonable that, say, the US can build more subs at the same time than Austria-Hungary, for example.
It was legitimately a thing. I can't speak about other nations but the US took steps 'tween wars to increase the capacity for submarine construction. Those included buying land near the Electric Boat works, expanding the sole periscope manufacturer, and the like. Key production bottlenecks for the US were periscope and optics, batteries and engines, with torpedo supply being a second-level concern (not that they didn't work right - no-one knew that - but that there weren't enough).
