No of P8s required...
Five does seem an awfully small number, however if it is SAR cover that's requiiored, look at what the Kiwis do with 6 P3-Ks, based in Auckland. The NZ maritime AOR is (IIRC) 35% bigger that the North Atlantic - granted, less traffic, moreover their P3s ahve been progressively upgraded to give them a pretty impressive overland ISTAR capability.
Size isn't everything...