I stumbled across an interesting article from the Nov. 27, 1930 edition of the Prescott Evening Courier (copied below) discussing golf in Hawaii, including the description of a 19th hole where the player would launch a drive into a volcano. The article notes that there were 20 courses on the four principal Hawaiian islands at that time:
Hilo - 7 courses
Maui - 4 courses
Kauai - 2 courses
Oahu - 7 courses (with an 18 hole municipal course being under construction)
Here are the courses that I know were in existence by around the time the article was written:
Ala Wai Municipal GC (Waikiki, Oahu) - possibly the municipal course that was under construction
Hilo CC (Hilo)
Kilauea GC (Hilo)
King Kalakua CC fka Schofield Barracks GC (Oahu)
Maui CC (Maui)
Mid Pacific CC (Honolulu, Oahu)
Oahu CC (Honolulu, Oahu)
Palolo Municipal GC (Palolo, Oahu) - another possibility for the new municipal course
Waialae CC (Honolulu, Oahu)
Waiehu Municipal Links (Wailuku, Maui)
That leaves the 2 courses on Kauai, 5 on Hilo, 2 on Maui and 2 on Oahu that are unaccounted for.