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?
Via Athanasios Karavasilis, Alcofribas, René Z.