The move from a Six-Six to an Eight-Eight Fleet is explained through the number of ships that can actually be ready. The answer estimates that an eight-ship category could normally provide four available ships, giving four battleships and four battlecruisers. A larger pool makes it easier to draw an extra ship without disrupting the whole maintenance cycle. Japan's nearby Pacific infrastructure is another reason offered for expecting eight fully maintained and worked-up ships.
Notable Point · 8:49-11:12
Why an Eight-Eight Fleet needs slack for maintenance
The Eight-Eight plan is linked to keeping eight ships ready while the rest of the fleet passes through maintenance and preparation.