Japan had four yards physically capable of building very large capital ships, but their supporting industry limited how many could progress together. The estimate is roughly three ships under active construction at any one time. Increasing that rate required investment beyond the yards themselves. New steel plants and other supporting facilities took time to build, so additional funding could not immediately remove the bottleneck.
Notable Point · 1:37:28-1:38:34
Why four Japanese yards did not mean four ships at once
Supporting industry could limit simultaneous capital-ship construction even when four Japanese yards had enough physical space.