At least one guy over at the Leica User Forum thinks
the answer is about 150,000.
IMHO owning a Leica wouldn't be quite like owning your typical Japanese DSLR/EVIL camera. The M9 is hand-built and tested and Leica are quite happy to service "historical" models. From Leica's website:
Depth of knowledge is as important as depth of field. Not only are Leica's worldwide repair centres constantly monitored and certified, but our factory technicians have a profound understanding of every last detail of every Leica ever made. Leica maintains a worldwide service network to ensure the legendary quality, precision, long life, and intrinsic value of Leica products for many years. Current or historical Leica models are maintained or repaired at the Solms factory or in an authorised workshop near you, by trained technicians using only original spare parts.
If and when I buy my first Leica (M10 perhaps) I would also hope I'd have bought not only my last Leica but my last interchangeable lens camera!
P.S. Mind you, with the price of Leica glass and the need to start a collection of same I probably wouldn't be in any rush to buy another body even if my bank manager let me.