How many times has the world/humanity/civilisation (probably civilisation at the largest) been saved from disaster by the actions of one person?
Unknown. It wouldn't surprise me for example if there were a few Cold-War crisises that are still essentially classified. But the technology has only really been sufficiently bad to have a chance to destroy humanity in the late 1950s or early 1960s. Even when the US and USSR both had nukes in the 1950s it is unlikely therw would have been enough to wipe out humanity.
Interestingly, I think the best case for human actions leading to world destruction take us back thousands to millions of years - where the butterfly effect leads to the action of each individual having a large effect on the future. Since the dead outnumber the living maybe around twenty-to-one, I figure they have the biggest chance of doing something that eventually has the effect of averting the end of the world.
Stanislav Petrov is a rather famous person (of course only on Lesswrong, not in the real world).
But there is another Russian who saved the world: Vasili Alexandrovich Arkhipov.
On this day in 1962, at the height of the Cuban Missile Crisis Vasili Arkhipov prevented the launch of a nuclear torpedo and thus a possible nuclear war.
It's strange that Petrov attracts much more attention than Arkhipov. E.g. googling "Stanislav Petrov" produces 101.000 results, "Vasili Arkhipov" only 9.040 results. By contrast searching for "Britney Spears" generates about 295.000.000 results. Sorta depressing.
Anyway, let this day be the Vasili Arkhipov Day.