When is your flight? Things can change very quickly! My guess is that, as usual for NW, the planes will be full but not over booked. They are generally good at filling every seat without going over! Now, a storm or mechanical delay can change all that. Just show up at the gate with spare time and ask the GA how full the plane is.

What fare are you flying on? A Y- or B-fare can be upgraded in AMS if seats are available in WBC using miles.