In Germany you can ask the authorities for the number of cars of a certain type registered. On www.rsturbo.de an enthousiast did that for a number of years in a row. Unfortunately for you it is only in German, but maybe nice to have a look at the pics.

I would rather like know the number of registered S1 cars in Holland, but either they don't know, or one is not allowed to know.

Ford Netherlands does not even know the number s1's sold.......