Many thinkers have considered this planet's carrying capacity. How many humans can live comfortably depends on how they live. Right now, we have about 7.7 billion. That extra .7 after the first seven is by itself a big number. Every tenth of a billion is another 100 million, or one-third of the US population. Each of us wants comfort, good food, a chance to do some travel, a good education, good housing, good clothing. Without getting into the optimal number of humans, it doesn't take long to get to the subject of the number of living humans and the rate at which they multiply.
The largest single nation is China, with about 1.4 billion. China can make a good claim for the title of the oldest continuous civilization on earth. It seems a credit to that government and its people that from 1979 to 2015, they tried hard to limit their production of babies. We watched the film "One-Child Nation" on Amazon TV and saw what a mammoth and difficult undertaking it was. Trying to limit the growth of population is a very difficult project. Humans have basic, very strong drives to reproduce. We cannot easily tame or direct that drive. The film makes clear the difficulties in setting the rules and in enforcing them to limit population growth.
If you want to help your country and the world and you understand the need to limit the supply of new babies, you might be asked to kill some healthy fetuses and some newborn babies. Even if you are fully convinced that there need to be fewer mouths to feed, extinguishing life in new humans could give you permanent nightmares. We are wired to increase life, not to limit it. With a one-child per couple policy, what about twins? Does one of them have to die? If a newborn is especially loved, who will leave the baby alive in a garbage dump?
Americans and many others in the Western world want cars, boats, planes. Also air conditioning, heat in the winter, and continuously available entertainment. Between the Netflix movie "American Factory" and the Amazon movie "One-child Nation", we can contrast the American ideal of fulfilment for each person in a long, happy, satisfied life with a clearer picture of life for many humans now living.
It can help to get a clear idea of how we live, what we might do to live more frugally and with less, and how to manage our moods and hopes about our current lives and our future.
