There is usually no clear cut rule on exactly how many people it takes to form a class action as each case is reviewed indpendently. However, for a case to be a class action, there is a "numerosity" requirement, which means the class has to be so numerous that it would be impractical to conduct the case by simple "joinder" of the parties. Those are two terms you can use in your own research if you are interested. Usually, a few people speak to an attorney regarding their potential claims and the attorney will be able to determine if a class action suit is warranted. Sometimes the potential class grows when more information is learned, similar to what you've noticed so far. You can find more information about an attorney through the attorney locator service on LawInfo's home page at:
http://www.lawinfo.com/. Some do offer free consultations.