Obviously I don't think 42 million actually support every aspect of ISIS and what they are doing... But even reducing that number to a couple million poses a massive threat and problem. There are somewhere between 30,000 and 100,000 ISIS fighters... plus those in foreign countries who have tried to emigrate. The establishment of a Caliphate means it is required of all Islamic people in the world to move there and live under its rule. Of course, many don't believe in the legitimacy of the Caliphate, but many do, and large numbers arrive every day. So assuming, about 75,000 ISIS fighters, I think it's pretty safe to assume quite a large number of supporters backing that group.