Gen 3:20 And calling is the human his wife’s name Eve, for she becomes the mother of all the living. Concordant Literal Version
By the time Cain went to Nod Adam and Eve had many other offspring not written in the Bible who were making many other offspring. The Scriptures just mention those important enough to mention. Adam was 130 years old when he had Seth after Abel was murdered. He may have had many other sons and daughters prior to Seth but Seth was a notable one. Genesis 5:4 said A & E had daughters too.
LLC, there couldn’t have been other people living since just prior to Adam and Eve being created the earth was chaos and** vacant of all life**
The Scripture is saying that up until the law of Moses was given that sin was not taken into account but that people were dying anyway. Why? Because Romans 5:12 just told us death passed through into all mankind.