You only need one. Aegis might be able to knock out a North Korea-class ICBM on a short trip from a limited amount of launch sites within North Korea, but by all accounts the Russian ICBM programs are the only thing that are actually pretty well funded and likely not completely hollowed out by corruption.

Add in the fact that MIRVs have been a reality for 70 years now, and hypersonic glide vehicles to evade anti-ICBM measures are too. Much like Reagan's Star Wars program, the existence of a perfect missile shield is pure propaganda.

In addition, even if casualties and strikes were extremely limited, let's say under a million people for some reason, the mere existence of a nuclear strike in the 21st century will undoubtedly change the world for the worse. We're all too young to remember the absolute paranoia of the Cold War, but restarting that kind of existential threat to all life on earth would bring about a complete destabilization of the world's society as we know it. I personally do not want to live in a world where genocidal extinction of Russians as retribution for a nuclear attack is seen as a worthy cause, you already have the MSNBC/Russiagate-crazed libs basically calling for it anyway.