Based on my very lay understanding if ecology, carrying capacity, and so forth, the earth is overpopulated. There are too many people for it to support sustainably, hence people in the know talking about the need for more and more techno-fixes (floating or underwater cities! nanotechnology! biotechnology! alternative energy! moon/mars colonization!) that basically further the reach of capitalist civilization into our lives. These solutions are often offered by people who acknowledge that there are too many people, but can't conceive of life without industrial capitalist civilization (or industrial anti-capitalist civilization), so the only solution they see is to hopefully raise standards of living, rely on expanding technology and hope people stop reproducing at some point. I quite disrespectfully disagree with all of that nonsense. I am uninterested in handing over my world to engineers and technocrats, and I don't think that more of what got us to the place we are now is a viable path to an anarchic future.
Does overpopulation present itself as an obstacle to creating anarchy? I think that depends on what you mean by creating anarchy. I don't think that anarchic living is compatible with mass society, so I would personally lean towards "yes" if we are talking about on going, sustainable anarchic projects (if there is such a thing).
On the other hand, more people allows for more co-conspirators and greater concentration of populace equals more chance for wildly out of control chaos. So in some senses, perhaps not. If you want a world with industrial civilization, and believe that that can work with anarchy, then you are probably inclined to want enough people to monitor and repair the automated factories that those weird green-tinged syndicalists talk about, though perhaps the number of people we have now is too many (but we definitely need enough that we maintain urban centers that are the loci of all the /good things/:art, learning, culture...)
So yeah, I guess that would depend on perspective.
