> you need to spend more than one defensive missile per attacking missile, because you might miss

It's almost always the case that you'll expend more than one interceptor per incoming missile as interception is probabilistic. In general, you can estimate the number of missiles you'll need to achieve a certain probability of interception with the following formula: 1 - (1.0 - sspk)^n, where sspk is the single shot probability of kill (i.e., successful interception with a single interceptor) and n is the number of interceptors.

Shameless plug here, but I wrote up a post about this problem: https://smu160.github.io/posts/missile-defense-is-np-complet...