It's about 50,000 men not counting air and naval base personnel.
Genda makes a couple of observations about having garrisons on the other islands, but there's not much else in the books. My best guess is that Kauai, Maui, Molokai, and Lanai probably have garrisons from battalion to brigade size, with the Big Island of Hawaii probably having another division-sized force; a brigade around Hilo, one on the north coast, and another on the west coast-and none of these alone would keep U.S. forces from landing to retake all the islands. Remember that the B-17s and B-24s flew in from the West Coast-dropped their bombs on Oahu, then flew to Kauai-where, despite the garrison, it was possible to covertly build an airstrip large enough for the bombers to land-and get them resupplied-submarine activity was not just interdicting the Japanese supply lines, but also distracting the Japanese Navy's interisland patrols enough to get some ships into the other islands-some old WW I four-piper destroyers that would be considered expendable would do-to get the Seabees ashore on Kauai, and probably Marine Raiders onto the other islands.