Those who believe humanity is at the thresh hold of catastrophic overpopulation are correct - IF we make only minimal changes to how we feed, clothe and shelter said population.
OTOH, those who maintain the Earth can support a much larger population are also correct, but only if we start changing, significantly, support for that population.
Certainly, simply expanding urban areas to handle population growth is not going to work, as this reduces agricultural lands, stresses the ecology for a large surrounding area, etc. etc. etc. This is due to the fact that, for all intents and purposes, we are supporting ourselves using 6000 year old methods, with (relatively) minor improvements through modern technology. However, when push comes to shove, we are still gathering in groups of inter-supported shelters (cities) while sticking seeds in the ground of surrounding areas, putting the modern version of composted manure on top, and waiting for it to grow into food we can eat; all the time dependent on - and vulnerable to - the vagaries of natural climatic forces. While it is true that since humans first started using determined plots of arable land to propagate domesticated plants, the ability of a single human to grow food has expanded from barely enough for his immediate family to feeding several thousand. But despite that advance, the base methodology is the same - and is starting to run up against the law of diminishing returns.
If a much larger population is to be sustainable, then we need to (literally and figuratively) get our feet out of the furrows and look to new means of food production, as well as how we use all other resources we depend on to run our societies. Food production could be increased dramatically. Using modern technologies, we could literally put food production within the cities themselves, eliminating, or at least greatly easing dependency - and stress - on the surrounding areas. But to do this, we would have to change forever what we use for food. Meat, and even real grains, fruits and cereals would become a rare (and prohibitively expensive) treat. Synthetic similies, made from processed yeasts and algae would become the standard fare for the family dinner table.
Water is composed of the 1st and 8th most abundant elements in the universe. But, for economic reasons, we deplete natural sources at rates far faster than they can replenish. OTOH, we have technology to take any source of wet, and turn it into drinking water, and recycle it at 99%+ efficiency. By our standards, such water is extremely expensive. But, then, at one time common black pepper was literally worth its weight in gold - now its less than an hours labor at minimum wage for a pound. One thing we have seen throughout history: economies --eventually-- (though usually involving much distress and upheaval) adjust to necessity/reality.
As such, overpopulation as an earth-destroying disaster is a boogie. We can and will adapt. Of that, I have little doubt. OTOH, I, personally, would not want to live in the society, or world, that would result from the changes needed to support 20 (or more) billion people on this one, tiny planet.
