If Kayin understood that Hashem wants him to rely on his intellect and be productive rather than simply rely on what earth gives him, he needed enough people to develop industries that will entice farmers to bring food in exchange for the goods. It was before the plough, so maybe he was making and sharpening cutting tools, axes, blades? Maybe medical and vet services? And then have research and teaching around that? I would say at least tens of people.
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