The estimates are all over the place, but the idea is that resources are finite including space. This is one of the reasons that there is a growing trend of anti immigration even among the left. (Not my opinion since I was an immigrant.)

https://www.issuesofsustainability.org/helpndoc-content/Club...

https://theguardian.com/environment/2012/apr/26/world-popula...

https://www.bbc.com/future/article/20220905-is-the-world-ove...