https://en.wikipedia.org/wiki/21-gun_salute

The practice of firing one gun for each state in the union was not officially authorized until 1810, when the United States Department of War declared the number of rounds fired in the "National Salute" to be equivalent to the number of states—which, at the time was 17. The tradition continued until 1841 when it was reduced from 26 to 21.

Deaths of presidents

See also: State funerals in the United States

A U.S. presidential death also involves 21-gun salutes and other military traditions. On the day after the death of the president, a former president or president-elect—unless this day falls on a Sunday or holiday, in which case the honor will be rendered the following day—the commanders of Army installations with the necessary personnel and material traditionally order that one gun be fired every half-hour, beginning at reveille and ending at retreat.

On the day of burial, a 21-minute gun salute traditionally is fired starting at noon at all military installations with the necessary personnel and material. Guns will be fired at one-minute intervals. Also on the day of burial, those installations will fire a 50-gun salute—one round for each state—at five-second intervals immediately following lowering of the flag.