How many people were in the world when the city of Enoch was founded (Gen 4:17)? How did Cain get a wife?

Evidently there were more than the ones specifically named in the text so far. After all, Cain had a wife, who would evidently have to be a near relation (a sister or a niece). We are specifically told in Gen 5:4 that Adam “begat sons and daughters.” Moreover, one must bear in mind that Adam was 130 when Seth was born (5:3), and that he lived another 800 years. Assuming that people aged very slowly then (as opposed to living out 800 years in extreme decrepitude), then both Adam and Eve could have been parents of hundreds of children. If Adam and his antediluvian progeny were typical, then each of those children could also have many children, so that the world could have many thousands of people in it after just a few hundred years, and not all of them would be near relations, either. It is not even clear that Seth, who was born when Adam was still a young, virile 130, was born after the incident with Cain and Abel, although I expect that was what the narrator intended, since that is the order in which the narrative proceeds. But if indeed Seth and others were born before Cain killed Abel, which is possible, then Cain might have had any number of cousins to live with the town of Enoch in the land of Nod.