is it one call per material? ie: if we have several sources of a full screen effect using the same material, does it only do one pass for all of them or does it do one per call?
Unity doesn’t do any fancy “combining” (it wouldn’t make sense either - since usually your image effects are rendered into different render targets).
So, Unity just executes the image effect scripts (OnRenderImage functions in them), in the other they appear on the camera. What you do in your image effect scripts - completely up to you. You could be querying databases for all we know 