This has changed since I was a controller.
It used to be at an uncontrolled airport only one IFR operation was allowed at a time. At a controlled airport the control tower would separate IFR operations.
When on a IFR flight plan (clear and 20) into a controlled airport I hear ATC tell me and other pilots that it's one at a time. Why can't tower accept some responsibility for arrivals in trail?
Example in the attached file.