Soviet policy once they got the ability to deal deterring damage to CONUS was to get a survivable second strike capability as they never had the luxury of first strike (so launch on warning or launch on attack), so they wanted to keep enough warheads after their retaliation.
Deterring damage itself was actually a fairly low number of warheads, dozens to a few hundred. It is doubtful with such a money-saving policy that they would actually commit to nuking everyone at once even when some countries are not involved at all.
The Soviets also completely rejected MAD as a desirable concept.