Only one plaintiff is required. At some point the plaintiff will seek to certify a class. If a class is certified, putative class members will have an opportunity to opt out. If they don't opt out, they will be included in the class. Occasionally, as part of the certification process, the order will provide that one must opt in in order to be included in the class (an "opt in" class), but that is rare.
Frequently, a class is "certified" for settlement purposes only. The settlement will include a release of the company and its officers and directors, and the company's goal then is to have as large/broad a class as possible. Except for such a "settlement class," the goal is obviously to defeat class certification.