Uwe, Although you might get away with one, I would buy two, and you will soon see if you need any more. The cans are small, but the bead is very thin. Airstream.com has always been fast at delivering to me. After a little practice (push, don't pull), and total concentration on the edges of the bulge of caulk, you should achieve beautiful results with this tool, without needing masking tape. Lewster uses this material professionally, and he may have a more accurate idea of how much will be needed.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."