Some would say that any level of population is undesirable, because humans cause environmental destruction. These people are usually greenie religious fanatics.
Others say that no level of overpopulation is undesirable. These people are usually religious fanatics who believe that it is God's will to create as many human souls as possible, regardless of the human suffering that results. Frustrated with skepticism about their belief in heaven and hell, they seem to want to create a hell on earth.
The argument normally goes 'If you think that there are so many people that bringing more into the world will increase suffering, then let them be born, and ask them whether they want to die. If they want they will kill themselves.'
This is a powerful argument which involves a sophistic twist. One more child born into an overcrowded world reduces the utility for the rest of the population, but it creates one more person, who wants to live. It relates to the value of a human life, and also to the notion of a soul. If bringing that extra child into the world is not wanted by the rest of the population, is it acceptable to stop it? Does something that doesn't yet (and may never) exist have a right to say?
More significantly, if an extra child in not in the interests of the whole, but is in the interests of the potential parents, is it acceptable to force or pressure those parents to prevent it? The Chinese government seems to think so. We have not yet seen a democracy come to this conclusion, but it may come about with the coming population explosion.