Amos, Shanghaiceltic et al.

I believe that warships of Nelson's day and thereabouts, held at least some of the shot for their cannon in "garlands". These were flat wooden racks recessed into the decks along the centreline of the ship. When not in action or at exercise, wooden lids covered the garlands. The garlands plus lids stood only a few inches proud of the decks themselves, allowing sailors to stand or walk on them, sling hammocks over them etc.

At a guess, ten or perhaps twelve rounds per gun could be carried in the garlands. Don't forget that shot is quite heavy. When clearing for action in a hurry, you would not want sailors staggering up the ladders each carrying a twenty-four or thirty-two pound lump of metal. No doubt more shot would be carried in the holds, but it is reasonable to suppose that after twelve broadsides, the battle would be won or lost.

I believe the concept of "ready-use" lockers came later in Queen Victoria's time, with brass cartridges and QF ("quick-firing") guns which could get through ammunition faster than the hoists could fetch it from the magazines. The ready-use locker could allow the guns to indulge in a minute's rapid fire in case of emergency.