On 6 August 1945, the B-29 bomber Enola Gay dropped an atomic bomb on Hiroshima. The United States would drop a second bomb on another Japanese city, Nagasaki, three days later. At that time, those were the only two functioning nuclear weapons in the world.
Today, on the 70th anniversary of the bombing, there are 15,700 warheads in various countries, according to ICAN.
Nine countries possess nuclear weapons, while a further five house them, and 23 others are part of nuclear alliances.
Source: ICAN
Since the end of the Cold War, treaties have lowered the number of nuclear warheads, as shown in this timeline.
Source: World Economic Forum (weforum.org)