"Nuclear-weapon states should negotiate and conclude a treaty on no-first-use of nuclear weapons against each other or make a political statement in this regard," Sun said.
China and India are currently the only two nuclear powers to formally maintain a no first use policy. Russia and the United States have the world's biggest nuclear arsenals.
I think you overestimate how many nukes it would take to cause the end of the world. Unless you mean "every piece of land is a radioactive wasteland" end of the world.
If just India and Pakistan were to go to nuclear war with each other, in their small localized region of the world, 27 million people would die from the carnage. The resulting nuclear Autumn would be enough to change agriculture and starve 250 million people worldwide.