Having more weapons is a more effective deterrent against an enemy who thinks he can destroy most of your weapons before launch. Suppose country A estimates it

Having more weapons is a more effective deterrent against an enemy who thinks he can destroy most of your weapons before launch. Suppose country A estimates it can destroy 90% of country B's weapons before launch in a first strike, and can spend $100B to get to 99%. It'd be highly motivated to do so. But if country B doubles its number of weapons, it would be beyond country A's resources to get to an acceptable level of destruction. That's one of many dynamics that causes arms races to go far beyond one weapon per target.