CVNs are cheap compared to oversees bases, and we have lost many of those in past years anyway. Any modern conflict requires air superiority to survive, and CVNs are the only way to effectively project air power in many areas of the world. Some overseas airbases have run into problems before by being denied access to the airspace of countries which lay in any practical route to a target. CVNs can move to change routes.
The number that we have takes into account: how many we want on station at any given time in various places in the world, how many might be in some stage of refit, re-supply, or repair at any given time, reinforcement reserves, and both goodwill and saber-rattling voyages.
As far as NK initiating military action, I suggest that we respond with all of our air and sea power to destroy their military might as quickly as possible, but don't put any boots on the ground in NK. No nation building! Let them suffer the consequences of their belligerence.
I actually wonder if they have any workable nukes. Their testing so far has had poor results. I always thought their nuke program was a bargaining chip to trade for groceries and oil, and only collaterally as a "we're too bad to be screwed with" scare tactic.