I was under the impression that multiple instances make the same number of drawcalls as the original prefab. I have tested this with a prefab of a unity primitive (cylinder) which has one diffuse texture material - one cylinder prefab creates one draw call, and multiple cylinder instances create one draw call in scene also.
So why then when I place multiple instances of a more complicated prefab, do the draw calls go up with each instance?
The prefab is a building with 4 materials and an animated, parented door with one material. There is a parented trigger box near the door which opens/closes the door and plays a sound during the animation, nothing too complicated. The prefab is dragged into the scene, not instantiated, and apart from the cylinders there are no other objects or scripts in the scene.