Upvote:6
Base on what you're describing you will enter the Schengen area only once, and you only need a single-entry visa for that.
In the the itinerary you're describing, you will enter the area in Amsterdam. Flying internally in the Schengen area does not count as "entering" the area; you're already inside before you take that flight.
For "Which Schengen country will you enter first?" you should answer the actual first point of entry in the itinerary you will enclose with your application -- that is, according to your current plans, the Netherlands.
It is understood that you may change your particular travel plans after the visa is issued. As long as the trip is still recognizably the same as the one you applied for (in particular, your main destination is still the same conference in Norway) this is not a problem. The visa you get will not be tied to entering the area at a specific point, even though you stated one in the application.