Though a class action is generally filed by one individual serving as the representative plaintiff, this person represents a bigger group of people. For a class action to proceed, there must be enough potential plaintiffs to demonstrate that it would be impractical to handle each case individually (Fed R Civ P Rule 23(a)(1)).
No specific number of people is required to file a class action lawsuit. Depending on the specifics of the case, there could be anywhere from several to several thousand plaintiffs. However, it is commonly thought that if there are at least 20 class members, the class action is more likely to be certified.