You don't need that many, but you aren't accounting for several factors. Firstly, missle defense, depending on the target some area have about a 20% success ra

You don't need that many, but you aren't accounting for several factors. Firstly, missle defense, depending on the target some area have about a 20% success rate for ICBMs. Secondly, secondary strike, if many of our sites are taken out by their misses, ground attack, or terrorist attack, we still need an arsenal that serves as a deterrent. Lastly, failure to launch, there maybe several missle crews that refuse the order, the soldiers on the button are trained and constantly studied to insure they will do their job. But when push comes to shove many people are unwilling to end the world as we know. In addition to that, some sites may not be able to authenticate the orders, communications maybe lost, or the missle may simple not fire. These are just a few scenarios that have to be accounted for, and when you do that, have a massive surplus only makes sense.