Which is the reason you want to have more nukes than they have silos - to be able to hit all of them at once in a surprise attack (also called "first strike").

Which won't save you against their SSBN… Granted, they have much less warheads in submarines than in silos, but that's still far enough to obliterate every city above 100.000 inhabitant in your country.

Indeed, which is why submarines are the linchpin of modern MAD. But they weren't always present; the buildup of nuclear arsenal started when the options were limited to ICBMs and strategic bombers.

Efficiency of submarine fleets were always hard to calculate, based largely on guesses about the adversary's true anti-submarine capabilities.