There is no exact definition as to what a "background" really is. I'd say that when you move from one screen to another, you have seen two backgrounds. I prefer to use the term "scene" for each new screen. The number of scenes are usually much higher in first person games than in third person games. You have to have a wider perspective in third person games, since it must be plenty of room for the character to move around in. Hence, the scenes in these games usually contain more details and objects to interact with. And, as have been pointed out previously in this thread, you can sometimes pan across the scene, which obviously makes it even larger.
I would also count a close up of an object or small location in a larger scene as a separate scene, while some may not.
Locations on the map, or places you can "travel to" is something different, in my opinion. There are games with plenty map locations, but few scenes, and games with lots of scenes contained in a small amount of map locations.
For some absurd reason, I used to count the number of scenes in the first adventure games I played, all of them from Sierra. I think that Leisure Suit Larry had about 80, while the King's Quest games had between 120 and 300. I'd guess that the average number of scenes for a commercial adventure game today is 80-100. But one has to keep in mind that the graphics are more detailed nowadays than in the early Sierra games.
3D games like Dreamfall is a totally different cup of tea. Since you move around seamlessly between rooms and locations, there is no actual method of establishing the number of scenes.