There is no limit to the number of possible class members. It could be hundreds or even thousands of people. All class members, however, must share commonality. In other words, there must be issues common to all class members, such as similar injuries, losses, or circumstances.
Class action lawsuits help to spread the costs of litigation across numerous plaintiffs. Legal fees are taken out of the total recovery, and plaintiffs do not need to pay to join a class action. Class actions also provide a convenient and efficient way of dealing with numerous, similar lawsuits.
A judge must certify a case as a class action before additional class members can join. Class actions are often filed over claims such as (alleged) personal injuries, fraudulent business practices, dangerous or defective products, securities fraud, employee discrimination, and environmental disasters.