A single person is generally enough to file a class action lawsuit so long as the class action attorney has a good faith belief that a number of other people were injured in a similar way. It is not uncommon for several people to be named as plaintiffs in the class action lawsuit.
The named plaintiffs (also known as class representatives or class reps) file the class action lawsuit on behalf of the unnamed class members (the so-called putative class).
At the appropriate time, when the class action lawyers move to certify the class, courts typically require that there be a showing of numerosity, i.e., that there are an adequate number of putative class members. Courts often differ on what adequate in terms of the size of the putative class, but typically anything over 40 is sufficient to show numerosity.