Currently, the population on earth stands at 7.4 billion. Despite the huge population of human beings, within the six realms of existence, there exist not only human beings, but a limitless number of other sentient beings as well.
Buddhist cosmology differs from the worldly cosmology that we know of. From Buddhism’s viewpoint, there exist an incalculable number of worlds in our universe, and countless number of sentient beings. Through their virtuous spiritual practice and good karma, sentient beings from other world(s) take rebirth into the human realm and are reincarnated in our world. If one day, our world is destroyed, humans would take rebirth in another world.
Currently, the universe which humans are most familiar with comprises the earth and moon, i.e. the planetary system. In Buddhism, the universe is bigger and much more than this material world that we know of, and includes the billion-world universe, a mind-boggling number of worlds and countless sentient beings.
(Extract from 2011 North-West University Q & A)