How many languages can one person speak?

Who hasn't wanted to master not just two languages but 10? Take Giuseppe Mezzofanti, a 19th-century priest who was said to be fluent in as many as 50 languages. Native speakers came from all over the world to test his abilities, and many left astonished. In Babel No More, Michael Erard investigates the legend of Mezzofanti and other linguistic prodigies, or "hyperpolyglots".
 How do they do it?