The official Japanese Metal Gear twitter has released graphs showing the amount of nuclear weapons owned in Metal Gear Solid V: The Phantom Pain on each platform. On them you can see how ownership has varied during the past 16 days (May 1st until May 16th).
Bringing the number down to zero on any platform will trigger the ‘nuclear abolition event’, a hidden cutscene. More information can be found here.
Source: Metal Gear Official Twitter (1,2,3)