Genesis 4:17 (NRSV)
Cain knew his wife, and she conceived and bore Enoch; and he built a city, and named it Enoch after his son Enoch.
The Genesis account is highly compressed history so we aren’t told exactly when he built the city. City in those days is probably not what we would think of as a city today. E.g Rome ~1000 BC had a population of ~4,400. So Cain’s city could have been perhaps 1,000 people, and probably less at founding.
Where did the people come from? They didn’t all have to be Cain’s descendants. Like cities today it probably attracted immigrants. But for arguments sake could it have been filled only with Cain’s descendants?
The Bible says that Adam and Eve had other sons and daughters without specifying the total number, but according to Josephus “The number of Adam’s children, as says the old tradition, was 33 sons and 23 daughters.” I personally met someone who came from a family of 20+ and there are other examples listed here. Given the long lifespans of those days 56 children is not unreasonable.
So let’s say Cain and his wife had only 20 children surviving to adulthood and this reproduces each generation. We get for each generation 2, 20, 200, 2000; so by the time of great grandchildren there are enough people for a small city.
So, @Reggie_O_Donoghue, it is quite feasible under a YEC view for Cain to have founded a city. We don’t need to postulate other humans alongside the line of Adam.