It is possible that Adam and Eve had dozens of other humans to populate the world soon after creation. Although there is no proof that there were other humans during the time of Cain and Abel, the Bible often hides and does not reveal its secrets so that the reader can figure them out on their own. For example, we know that when Adam and Eve bore Seth Adam was 130 years old. And we know that the Bible does not always give a complete list of people's names. Adam and eve's genealogy is not complete. In Genesis 5, for example, the list only lists a single person from each generation, spanning from Adam to Noah. Similarly, chapter 10 only lists the male descendants of Noah, but not the females. It is possible that during those long 130 years, Adam had more offspring, male and female. The Bible only lists Seth because he was an ancestor of Abraham. Thus, the Bible not listing other people's names at the time of Adam and Eve is consistent and should not prompt us to think that other people did not exist at the time.
Genesis 4:14 supports my interpretation. Cain says “whoever finds me will kill me.” If we accept the idea that there were other people besides his parents and Abel’s wife's, it makes sense that Cain would be afraid of these people. Also, Cain goes to 4:16 and “dwelt in the land of Nod.” If there weren't other people, why is there a land of Nod? He then builds a city. If only a small family existed, why build an empty city? This idea is supported by the fact that Cain and Abel had vocations. For example, Abel herded sheep and Cain tilled the soil. If there was only a small family, hunting would have been sufficient. Why herd sheep if not to feed an abundance of people.
Lastly, in verse 1:26 G-d said “Let us make man in our image, after our likeness” and in 2:7, “Then the L-rd G-d formed man of the dust of the ground.” We could understand “man” here as “humans” in the plural. In the Guide (part 1, chapter 7), Maimonides describes animals who resembled the physical features of Adam. In his book, the Science of G-d, Gerald Schroeder described these Homo sapiens (pre-Adam hominids who lacked a neshama) to be “human-looking creatures,” “animals with human shapes but lacking the neshama.” (See The science of G-d, at 123.) When the Mishna mentions the “masters of the field” it is denoting the pre-Adam people (who had the intelligence to perform the skills such as agriculture, 2000 before Adam).
