That is a good question. I don't know if there is a cut and dry answer like for carriers.  There are 10 'official' aircraft carriers and another 9 ships that could be technically called 'carriers.' As I mentioned, there are only 4 E-4B's total and I have never seen 2 up at the same time. These are essentially airborne bunkers, designed for continuity of government, global communication and command and control of entire military in the worst of worst situations