Contingency plans for responding to a decapitating strike will do the rest.
https://en.wikipedia.org/wiki/1983_Soviet_nuclear_false_alar...
> Shortly after midnight, the bunker's computers reported that one intercontinental ballistic missile was heading toward the Soviet Union from the United States.
> Petrov considered the detection a computer error, since a first-strike nuclear attack by the United States was likely to involve hundreds of simultaneous missile launches in order to disable any Soviet means of a counterattack.
Now I wonder if one missile fired by a submarine would be similarly dismissed as computer error...
I highly recommend the book Command and Control.
Not that the difference practically matters. You only need enough to saturate the defences plus one to cause a very bad day for everyone involved.
It's almost as if both were problems that need to be tackled in different ways, not issues to be used for whataboutism.