Bach's "Well-tempered Clavier" consists of two books, each containing the same number of fugues.  In the first book, instead of having (as usual) one subject, one fugue has two and another has a prime number; in the second book, two less than half the number of fugues in it have two subjects, and one has the same prime number as before, which is a measure of the whole number of fugues, and also of the whole number of subjects diminished by eighteen.  If the whole number of fugues were increased by one, one-seventh of the result (which is an integer) is less by two than one-third of the whole number of subjects in the first book.  How many fugues are there?