The comparison sizes a class around its operational and refit rotation. Under the assumptions given, five ships usually provide two available hulls, while six provide two reliably and make a third easier to produce when needed. The extra hull does not mean keeping six complete crews aboard six ships at all times. With ships in refit, the comparison is between fully crewing three of five ships and four of six.
Notable Point · 2:53:27-2:54:41
Why six ships provide more than one extra hull over five
A sixth ship improves the operational rotation and the ability to make a third hull available.