Graham Cansdale is a professional translator at the European Commission in Brussels. He is considered fluent in the 14 languages he uses in his work, but he has studied other languages. Cardinal Giuseppe Mezzofanti was a linguist who died in 1849; he was said to have known between 40 and 72 languages. Is this even possible?
There are millions of people, even in the mostly monolingual US, who speak more than one language at home. Competence in three languages is not unusual, and we've all heard stories of grandmas and grandpas who had to master four or five languages on their way from the old country to the new. In India it is common for people to go about their business every day using five or six different languages. But what about 10, 20, 30, 100 languages? What's the upper limit on the number of languages a person can know?
Michael Erard, in his fascinating book Babel No More, travels around the world in search of hyperpolyglots, people who study and learn large numbers of languages. He sheds light on the secrets of their success, and explains why it can be hard to put an exact number on language knowledge.
Mental_floss introduces us to seven hyperpolyglots, although how many languages they know depends on the meaning of "know," as there are different levels of fluency. Of course, only a polyglot would really understand how different those levels are. Link
I used to teach my students a few words in Klingon. Alas, there's not a lot of interest in the language of love these days. Now I teach them Ewokese instead.