According to Nukemap, a fabulously horrible calculator of the destructiveness of nuclear explosions, a five megaton nuclear weapon, such as those on China’s nuclear missiles, will totally devastate an area of 1880 km2.
According to The Physics Factbook, the land surface area of the Earth is 148,300,000 km2. This suggests that it would take 78,883 five megaton nuclear weapons (a total blast of 394,415 megatons) to absolutely devastate every inch of land on the Earth.
Not that this is required to end human life. After World War Two, US scientists estimated that as few as 100 large nuclear explosions around the world (a total of up to 10,000 megatons) would be enough to end all human life, because of atmospheric poisoning with radiation.
In actual fact the world nuclear arsenal is apparently about 9,000 to 15,000 weapons of varying sizes., I have not been able to find information about the total megatonnage of these weapons. Some are undoubtedly less than a megaton; but many are much more. It seems likely that more than 10,000 megatons of weapons are available.
Since the 1950s, research into the destructiveness of nuclear weapons has tended to increase the estimate of the long-term danger posed by their use. Nuclear winter and the ecological destruction attendant upon mass destruction of habitat and radiation effects would multiply the risks to all complex forms of life.
The utter unsurvivability of any general nuclear war — regardless of where people live on Earth or how they try to protect themselves — has been well known in military circles for many decades, and it is this knowledge that protects us from any step towards the use of nuclear weapons.
After a general nuclear war, there will be no ‘Mad Max’ scenarios, with colourful barbarians scratching out a perilous existence in a post-apocalyptic desert. There won’t be anything like the grim world shown in “The Road”, or in “The Book of Eli”, or any other post-apocalyptic movie that shows a planet with people on it. There will be nothing bigger or more entertaining than cockroaches.