This could’ve been a Zeitgeist documentary segment
Many companies in the capitalist system require an “infinite” supply of expendable workers, paid below the living wage. So, if we continue operating the anti-economy, we may require more people than Earth’s carrying capacity.
In nature, each specie has a population limit. When population is close to that limit, it stagnates. This limit can be increased by spreading and creating new breeds, however, that extended limit can’t go to infinity because of practical limitations of the environment.
A Resource-Based Economy can keep a large, but limited number of people on Earth, while using less land and resources. How the population limit works is similar to nature.
So, we do not need more or less children, but rather, a balance to make sure the population will not consume too much resources.