There are 10 seazones on the board, (DEI, coast of China, phillipines, and Japan) where Japan can be convoyed.
Meaning in general, you only need a handful of destroyers to keep them clear. If you’re getting sub spammed, build destroyers and use your aircraft to clear these zones. Even if you wanted to clear all of your zones, you would only need 10 destroyers to do so.
80 IPC’s is just over one average turn of income for Japan.
Playing it safe, I would say 2-3 destroyers near Japan, and 2-3 destroyers near Malaya, should give you all the coverage you need.
Remember that your opponent can only flood you, if you don’t have enough DST to counter attack most/all zones.