Your question is a little unclear. As asked, the answer is "one" -- you only need a single "class representative" to file a lawsuit on behalf of a putative class. You cannot do this without a lawyer because you cannot legally serve as the attorney-of-record for the absent class members, nor would you be an adequate "attorney" to represent them. You may have intended to ask, "How many people must be in the class?" There is no bright line test, but as a general rule the fewer members of the putative class, the more unlikely that it will be certified as a class action. It very difficult to win class action certification if the class has fewer than 30 people.
All the answers seem to be right but your question also seems to be more practical. Let me put it this way. You only need one person to start a class action lawsuit but for the court to "certifiy" it as a class action (which is the court saying that your case actually will proceed as a class action), you have to have what the law calls "numerosity." That means you have to have enough people who have been affected by the legally wrong act to make it more convenient for the court to treat it all in one case instead of having each person file their own lawsuit. Often, the quantity depends on the court, the state, the case, the circumstances, etc. In some states, there are cases that have given some general guidelines (like in Ohio it would be at least 40 people who could be "members" of the class) but in many states there is no specific number and it will just depend on the circumstances of the case. Class action cases can be extraordinarily difficult for the attorney to handle for you and they can take a lengthy time to get through the court process or to get settled. Most of them settle without ever having to go to trial.