+1 and make it an optional setting.
For example, Media Pro gives users the choice to include subsets in the count (in parentheses) or not. ("Catalog Set" is the MP name for a Collection.) MP also allows users to choose whether they want to display a "union" or "intersection" of the selected collections' contents. See attachment. (This is a a drop down menu opened by clicking an icon in the header bar of the Catalog Set panel.)