What's the maximum number of times a year i can white my teeth?

Depends...: Usually teeth reach a certain color and do not get any or much whiter. If you drink lots of coffee etc, and notice darkening, then once a month touch-up with your dentist made custom trays and low concentration whitening solution, used for a short time period (1hour or less) is typically not a problem, unless you have sensitivity.

No maximum: You can whiten your teeth in multiple treatments throughout the year. I would be cautious however of sensitivity which is a normal side effect of the whitening process. As always, it is best to make sure a dentist keeps a check on your progress for your own well being.