The answer is 31. It takes merely this few silver atoms to legibly write RUB. Not with a pen, of course. A special technique must be deployed to manipulate the individual atoms. The Chair of Physical Chemistry I, headed by Prof Dr Karina Morgenstern, owns a device that has this capability: a low temperature scanning tunnelling microscope. It images metal surfaces with atomic resolution; in addition, it can also be used for moving individual atoms around.