I am sure this confusion over what makes a 'new design' or a 'different car' is why they just stick to 'nameplates' now. For example, you just count the total number for each model - Galaxy, Squire, Zephyr, Scorpio, Mondeo, Fairlaine, Mustang, etc etc, regardless of whether they share a design or components.
Same with the Golf - you just count 'Golf' models, while the other Golf-based models such as the Jetta, Golf Plus, Touran, Eos, New Beetle, Audi A3 & A4, SEAT Ibiza, Skoda Favourit, etc etc are counted separately, even though they are the 'same' car underneath. It just gets too complicated and impossible to follow otherwise.
Probably just as well. The Beetle was never sold by the same name over its production life and was known by a variety of official names - VW 1200, VW 1300, VW 1300, Superbug S, Superbug L, VW 1600, Beetle, 1302 and 1303 (UK), Superbeetle (US), let alone all the non-English names such as Kafer, Maggiolino, Coccieelle, Fusca, Vosvos, Boble, Kever, Cucarachita, Kodok, etc etc in whatever country it was sold. And of course, Beetle in the lastfew years of its US life, and in 1976 in Australia.
There is a list of the Best-Selling Automobiles at
http://en.wikipedia.org/wiki/List_of_bestselling_automobiles