Q: The Urantia Book states the Milky Way forms the center of the superuniverse of Orvonton. Has anyone determined how many similar galaxies are contained in Orvonton?

A: I can only refer you to the following quotes:

(12:2.3) In the not-distant future, new telescopes will reveal to the wondering gaze of Urantian astronomers no less than 375 million new galaxies in the remote stretches of outer space. At the same time these more powerful telescopes will disclose that many island universes formerly believed to be in outer space are really a part of the galactic system of Orvonton. The seven superuniverses are still growing; the periphery of each is gradually expanding; new nebulae are constantly being stabilized and organized; and some of the nebulae which Urantian astronomers regard as extragalactic are actually on the fringe of Orvonton and are traveling along with us.

(15:3.4) Of the ten major divisions of Orvonton, eight have been roughly identified by Urantian astronomers. The other two are difficult of separate recognition because you are obliged to view these phenomena from the inside. If you could look upon the superuniverse of Orvonton from a position far distant in space, you would immediately recognize the ten major sectors of the seventh galaxy.

It is an exciting time we live in, when the Hubble Telescope, and even some earth-bound telescopes are seeing farther and farther into the vast reaches of space. It could be that in times to come, and with even more powerful telescopes, the exact numbers and locations of galaxies in Orvonton might be discerned.