We should show the total number of items in a list, a tree or, as a matter of fact, any item view in the bottom right corner, on the status bar. We also must show the number of currently selected items there - not showing this is a major usability issue as you e.g. don't know how many items you're going to delete.
We did have code to compute and show these number during 5.0 development, but it was deactivated as it prooved to be a major performance issue. We should revisit that code and either find a different way to compute this. An idea could be to delay computing at least the "total" number until the entire view is rendered and accessible. We could show a dash "-" while the numbers are computed.
Please refer to the attached screenshot for the exact positioning and message; the text should read "xy <type of item>, # selected" (e.g. "150 assets, 0 selected" or "62 contacts, 3 selected").
Please note that this information should also appear in any chooser offered by content apps.