Here is a wild guess. "Billions and billons" of stars orbit the center of the Milky Way Galaxy. The graitational center of mass of the Milky Way Galaxy is occupied by a supermassive black hole with many other objects orbiting it closely. That black hole might count as a "world" of sorts.
All of the billions of stars in the Milky Way Galaxy orbit around their common center of mass. But since there is a super massive black hole in that center, perhaps formed from many stellar mass black holes which spiraled down into the center and merged together, someone could claim that all of those stars orbit around the super massive black hole.
And thus all the planets which orbit the stars that orbit the center of the galaxy, and all of the moons which orbit those planets which orbit those stars, could be considered to orbit around the black hole at the center of the galaxy.
And of course astronomers have now detected rogue planets in interstellar space, planets which don't orbit any star but only orbit the center of mass of the galaxy. And of course, for every object of planetary mass, many objects of lower mass must have been formed. So there should be many objects with masses in the range of moon masses orbiting around the center of the galaxy.
So if somone wrote poetically of the supermassive black hole as a "world" and of a million "moons" orbiting it, they would actually be using extreme understatement in the numbers of "moons". There should be hundreds of billions, even trillions, of astronomical objects directly orbiting the center of the Milky Way Galaxy and the super massive black hole in that center.
And the same should be true of countless billions of other large galaxies in the universe.
So I am not certain whether Sagan ment the many moon mass objects which should be directly orbiting the center of the Milky Way Galaxy and the supermassive blackhole there, or if Sagan ment Saturn with probably a lot more than a million ring particles. A lot more than even the "billions and billons" Sagan often wrote and spoke of. This source estimates there are 3 times 10 to the 16th power ring particles:
Or maybe Sagan thought of something else which could be poetically described as a world with a million moons in that passage.