Then I repeat that the claimed not-literal number is is few in comparison to the billions of humans who have lived.
This argument is fallacious. You suppose that the number of Messengers and other Holy Teachers is (or ought to be) directly proportional to the number of human beings. This is not an accurate reflection of many realities in our world.
It would be equivalent to suggesting that because our solar system has eight (or nine, depending on whether astronomers can settle on what Pluto is), each planet ought to have its own Sun. Or that because there are 320, 330 million US Americans, there ought to be 320, 330 million senators or representatives in the House.