OK the better question is how many flyable aircraft are fitted with Mode S transponders. My current semi-educated estimate is over 110,000 with around 3,000 changes worldwide in any one given month.
The 370K odd US allocated codes and the 20K UK allocated codes include many many thousands of Gliders, Balloons, Microlights which are never likely to even be equipped with Mode C, let alone a broadcasting Mode S. Adding these to the system, and consequently tracking these aircraft sales and re-registrations would really divert the task from the important tracking of changes to Airliners, Biz Aviation and ADS-B aircraft that will be picked up often by the masses. Tracking and updating the re-registration of a non-radio equipped Piper J-6 Cub operating from a farm strip in the town of Truth or Consequences, New Mexico (just because the FAA publish a code for its future use) is not going to help anyone.
