There are 4x5 slots in the inventory and 4 items on the buttons, for a total of 24 item slots.
Deku sticks 1, nuts 2, slingshot 3, bombs 4, boomerang 5, din's fire 6, bombchus 7, lens of truth 8, bottle 9,10,11,12, farore's 13, nayru's 14, bow 15, hammer 16, iron boots 17, longshot 18, hover...