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I usually here the opposite, people claiming that we actually have enough and can host more people.

It depends on what standard of living we expect and how much of the environment we are willing to destroy. We don't have the resources for everybody on earth to live like an American but we do have the resources for everybody on earth to live like a North Korean.
Land is also a resource and even rich nations like Germany lacks it and could not host a population that wanted a suburban lifestyle like the US and Australia.

We also lack the rare earth materials to transition to green energy using the battery technology we currently have, there are many other issues with green energy right now as well, so you would have to accept global warming if you wanted to increase the population size.