This was an interesting question, because Dr. Boli did not know the answer—or at least he did not know the whole answer. Part of the answer was easy. The patterns are not randomly generated: they come from various old books and catalogues. They do often repeat, but you would need a very good memory to recall one specific marbled endpaper.
But to the question of how many there are Dr. Boli did not know the answer, so he set his secretary to counting. Here is the census of backgrounds:
77 marbled endpapers
72 tile patterns from various tile manufacturers
48 miscellaneous other backgrounds (such as wallpaper patterns, cloth bindings, fabric prints, and so on)
That makes a total of 197 different backgrounds so far, which is enough for a pleasing variety. The collection grows every time we find something in an old book or catalogue that would make a suitable background.
The resources for building a collection are readily available. Old books at the Internet Archive commonly have marbled endpapers. Catalogues from tile and wallpaper manufacturers are full of patterns.
And, by the way, none of the patterns on this page are in Dr. Boli’s collection yet. It will take a while longer to exhaust the inexhaustible riches of the Internet Archive.
It takes a little practice to crop one of these patterns so that it repeats correctly—perhaps five minutes or so of practice.
Marbled endpapers, of course, cannot be tiled so neatly, but the complex ones are random enough that the seams are not offensive. In that sense, the marble patterns do in fact come from a random generator—namely, the constantly swirling colors on which the paper was laid.