In regards to how many languages one can learn, my view is simple; the more you learn the easier it gets. So someone who can already speak four or five languages fluently (check the articles on definition of  ‘fluency’) will certainly find it a lot easier to learn another four, and even easier, another four after that.
It just becomes a matter of time , availability and determination at that point.
However, I find that there is a certain limit to vocabulary capacity, where at certain point it becomes more difficult to learn and retain new words