I've seen estimates on Earth's carrying capacity range form less than a billion to a trillion, all just depending on what you think technology and science can do in the future (think massive arcologies, super efficent AIs, giant space solar panels, and advanced molecular nanotech), and how resilient nature is.
What can be said with confidence is that the Earth currently grows enough food to support 10 billion people, when some people back in the 60s and 70s predicted massive starvation and shortages would have happened long before that.
What can be said with confidence is that the Earth currently grows enough food to support 10 billion people, when some people back in the 60s and 70s predicted massive starvation and shortages would have happened long before that.