Konami has provided a new update detailing the current number of nuclear weapons owned in Metal Gear Solid V: The Phantom Pain. The red area of the graphs shows the changes since the last update on June 9th.
Looking at the graphs you can see that after having gone down for a couple of days there has been an increase on each platform, with the exception of the PlayStation 4.
The current numbers, as of June 26th, are as follows:
Xbox One: 153
Xbox 360: 305
Since last update:
Number of nukes went up
Number of nukes went down
Source: Metal Gear Official Japanese Twitter (1,2)