The goal of the current US missile defense system is to be able to counter 10s of incoming warheads by firing 200+ interceptors. The last I heard, only about 25 interceptors had been deployed. Bear in mind, however, that everyone with modern ICBMs uses MIRVs, so a single missile will release 5-10 warheads.
Honestly, the system is mostly meant to counter the growing threat from North Korea. Trying to defend against a real attack by Russia/China/almost any other opponent simply isn't realistic with current technology.
Yup. And there is technology to spot naive decoys (e.g. IR signature), and technology to make decoys more sophisticated. The ABMs have to hit the MIRV before the warheads deploy or knock out every potentially damaging warhead.
Russia and USA as the nuclear triad are in leagues of their own, with China/UK/France lagging far behind. China deliberately does not have a MAD capability or strategy. With an attack from an adversary like NK/Pakistan, we would hope we could throw a flock of ABMs at their missiles as fast as they could launch them and immediately blast the shit out of them in retaliation.
China might well be able to get stuff through, but we could limit the damage.
Russia is another ballgame. In an all-out nuclear assault, we could hope to knock out a few warheads here and there, but the numbers are grim. We would be facing a large proportion of our population centers looking like Hiroshima circa September 1945.
So yeah, North Korea is a leading threat, since they are led by a madman who could start a new bloody war in the Korean Peninsula that would devastate our closest allies in the region and pull in China and Russia, and may someday soon if unimpeded be able to actually develop a damaging missile or two that could reach our western shores, but Russia is an adversary led by a murderous autocrat who gives few fucks and could potentially push buttons that would make us a smoldering wasteland in a matter of hours.
Keep in mind that decoy RVs (inflatable mylar targets) are cheap. Overwhelming your adversary's sensing and response systems is a classic tactic, and not just in military combat.
