If C&C is destroyed, Russia cannot do a coordinated launch of its full arsenal. If C&C in Moscow is taken out, it will be up to people in command bunkers in Russia to decide on taking own action, fully knowing that random fire at will launch is futile against a seriously NBC prepared adversary.
Only silo based ICBMs are kept in state of high readiness, and can be fired "with a press of a button." That limits targets by range, number of delivery vehicles, and whether the warheads have their own individual last stages. Russian has 3 major silo based ICBM field, each built with 1 major target in mind: 1 for Europe, 1 for the US, and 1 for China. We don't know how many of missiles will be retargeted, or how many can, but we are certain that a lot of ICBMs employed, say, in the Russian Far East, cannot reach most of the US even with a single warhead.
The highly improbable scenario of Russia being allowed to launch all of its ~1000 at short notice, and launch them only at densely populated cities is predicted to cause 50M deaths in USA without BMD, people taking to nuclear shelters, and no civil defence defence response.
In reality, it's practically impossible to successfully fire their whole arsenal without prior notice, and fire it on designated target only in Europe or North America.
The realistic estimate is that the US will face 300 to 500 warheads.