How many languages is it possible to speak well? The 18th-century Italian cardinal Giuseppe Mezzofanti was said to have known 70 languages, and apparently once learned Ukrainian in two weeks. A handful of other people over history have been described as knowing 50 or 60 languages.
Studies, however, suggest that most people can't cope with speaking to a high level more than seven or eight languages at any one time. A hyperpolyglot is defined as someone who knows six languages or more.
An article in last week's Observer reviewing the book Mezzofanti's Gift by Michael Erard was the source for the above information, and much more. Erard travelled around meeting people who had learned several languages, and looked into scientific studies that examined the brains of rats. Some individuals do seem to have talents for learning lots of languages, but Erard also discovered that the language faculties of the brain can be improved the more they are used, just as muscle power increases the more you exercise.
Here's the article.