Hi Joachim, as both a coffee lover and a barista I would say "How much as you like!", but of course there's a limit that is considered "safe". This limit, similar to what happens with alcohol levels, of course depend by many factors: age (of course a kid cannot drink coffee!), healt issues, psychological sate, and so on.
As an average, The U.S. Food and Drug Administration considers 400 milligrams (more or less 3 cups of brewed coffee or 4 double espresso) a safe amount of caffeine for healthy adults to consume daily. Above this level per day, people may experience nausea, headaches, sleep difficulties or increased anxiety.