I always have a full fleet on each side of Honshu, around my westernmost/easternmost province. Granted, I've only played Chosokabe and Shimazu, so those fleets basically protected the west entirely, eventually. Though all the straits and bays in the inland seas between Kyushu, Shikoku and Honshu are a nightmare early on in both campaigns. I had 4 fleets for a while, to be sure not to lose trade income because of surprise wako pirates and instantly go bankrupt.