I hope that Americans have enough missiles to shoot down any target with North Korea. But such regimes in NK usually based on lies. I'm not sure their missiles can fly.
No SAM system is 100% effective and accurate. So there is always a chance of a North Korean missile landing on the target. You can place as many SAM batteries as you want, but you can't just eliminate the risk. And regarding the quality of the North Korean missiles, a few random failed tests don't hide the fact that they have one of the best missile technologies among the third world nations.