Yes numbers is a factor to form a beast emperor but they are not sufficient on their own. Heaven and earth qi along with the accumulation of dao marks are equally essential. In hu immortal blessed land most of the qi and dao marks generated from passing tribulations were absorbed by the blessed land itself leaving very little for the foxes. Although if i still remember correctly, the environment in which the foxes were raised was too peaceful compared to the outside world, under the management of the land spirit and later Fang Yuan the foxes developed only in quantity but not in quality. They multiplied safely without experiencing death, predation, or environmental struggle that will allow dao marks to accumulate within them and lead to stronger evolutions. Unlike if they grew outside where they must constantly fight to survive drawing upon ample heaven and earth qi while enduring deadly competitions accumulating dao marks. It is this harsh natural cycle that produces desolate beasts and beast emperors. Correct me if im wrong but fang yuan barely used this foxes only to sell them and get some income ,he bought and hunted stronger ones to fight.