Don't know how many but I would bet it is a lot more than people think.

Not related to double rifles but you will get the idea

I live near and are friends with the Australian Hornady importer. When ever a new shipment of Hornadys arrive it never ceases to amaze me the huge number of 375 and 458 projectiles as well as odd balls like the 348. Where all the 375s and 458s are I have no idea but they obvioulsy out there. I bet double rifles would be a similar situation.