They can. What counts is how many missiles get through. Some will, some won't. That is what should prevent an enemy from even trying: he won't know which missiles will get through, so his military plans will be useless and what comes back will make his military plans irrelevant anyway. A further complication is that the missile defense would have to be neutralized before the main attack and that will provide enough time to make the necessary response and put an end to the threat. There will be some successful strikes; that is impossible to stop completely.
I wasn't aware that NK is prepared to launch more than one.