I would suggest having the vendor clean the sensor (assuming they can do this) to their satisfaction and yours. You've then established a baseline. Then track your usage (shutter counts and lens changes) over a period of time and see when and how quickly the dust returns.
If you shoot 25 shots with a prime lens indoors, and then seen dust, I'd say there is something within the camera generating it. If you shoot outdoors with a 28-300 zoom, it may be coming from the lens. If you change lenses every 5 shots, that could be it.
Once you get an idea one the circumstances, then you can go back to the vendor, or Nikon, if appropriate. But you need to start with a clean sensor. (Dust on the mirror doesn't count!)