How many times has the world/humanity/civilisation (probably civilisation at the largest) been saved from disaster by the actions of one person?
Zero times: that has never happened.
Are you asserting in the cases of Petrov and Arkhipov that events would not have turned into full scale war or that you think it is likely that additional events in the chain could have prevented that? If the first, I'm curious as to your logic. If the second, how proximate a cause do you need before you think you can speak of someone saving 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.