They would definitely fire more than 1. That’s part of the ground based mid course defense system. A single missile from GMD only successfully intercepts about 57% of the time, and this is in highly scripted training scenario. If you throw in all the unknown variables of an actual enemy ICBM I bet that percentage drops even lower. But if you do the math, if they fire 4 interceptors each with a 60% chance of hitting the target the probability of hitting 1 ICBM goes up over 97%. Although many people argue this is overly optimistic and it’s really still more like 50/50. .

But then if we can’t intercept mid course, we also have a different defense system for when the warhead is in its terminal phase. This presents quite a few challenges though as it typically only takes about a minute once the warhead re enters earths atmosphere and its traveling very fast. Although the path is more predictable at this point but you gotta do it immediately so the explosion happens way above earth. we also have another missile defense system for intercepting during the boost phase on its way up to space.

Have you ever read about what happened in 1983 in the Soviet Union? One of their nuclear defense stations detected that the US launched a ICBM, plus 4 more behind it. The station operator lucky thought it was mistake and somehow quickly confirmed that it was. He was supposed to send that information up the chain immediately which more than likely would have resulted in the Soviets launching back at us. Then we would have legitimately launched ours at them and full scale nuclear war would have happened. Because of how stupid the old Soviet system was he was never rewarded for this because that would have meant admitting their scientists/engineers built a shitty missile defense system. So he was reassigned to a less important position lol.