The maximum level is the level at which a) the average sentient being of that generation can be expected to live a net positive life, b) the addition of another does not reduce the positivity of other lives, and c) the individual being itself would live a net positive life. What is considered a net positive is its own question since pleasure exceeding suffering is subjective, but there’s a strong argument to be made that there is an increasing net negative, and that’s not nearly limited to the climate change argument (in fact that’s probably one of the weaker angles one can take).

You can also go sliding scale, though you’d have to compete with the eugenics argument (which is possible), and say that some children are worth bringing into the world and others are not. For example, huge net negatives would be someone who sucks up so many resources that they make the average human life worse or someone whose circumstances make them far more likely to live a qualitatively poor life.