The US military calculated how many nuclear bombs are needed for a particular city, depending on its area and building density.
For the destruction of the six largest settlements of the Soviet Union, the Americans would need six nuclear bombs. In total, in order to wipe out the entire country from the face of the earth, the US needed 204 nuclear charges.
The leadership of the USSR was aware of the plans of the Americans. Strictly speaking, this led Stalin to start developing his own atomic weapons.