I'm not sure if you're seriously looking for an answer. If you are, it's actually rather obvious: As long as there is a piece of unclaimed land, it is sufficien

I'm not sure if you're seriously looking for an answer. If you are, it's actually rather obvious: As long as there is a piece of unclaimed land, it is sufficient for a single person/organization/state to decide at some point in time to claim it for themselves, and from that point forward it is likely to stay claimed forever. Over the course of human history, it is exceedingly likely that every piece of at least marginally potentially interesting land is claimed at some point. Furthermore, only unused land can truly be no man's land, which is why it is a bit curious why you'd be so interested in no man's land in the first place. (As soon as you go there and do something remotely interesting with it, even if it's just camping there, it stops being no man's land and starts being de facto your land, until somebody challenges you to it.)