Jennifer Waid writes, "Myself and some other coworkers were discussing the Old Testament, and we are interested in knowing where Cain's wife came from. Did God create other men and women and it is just not in the Bible? Also, did Eve have other children besides Cain, Abel and Seth? Thank you, Jennifer."
That is certainly a subject which provokes questions. However, the answer(s) are not so very difficult to find, if we will let the scriptures answer them for us.
Actually the answer to "where Cain's wife came from" pretty much answers, or helps to answer, the other questions which you asked. We are told, in Genesis 3:20, "Adam called his wife's name Eve, because she was the mother of all living." Another point is the fact that God had said to Adam, "In the day that thou eatest (of the tree of the knowledge of good and evil) thou shalt surely die.
Adam transgressed the commandment of God, and thereby sin entered into the world, and death by sin. He and Eve did not die physically for many years, but they died spiritually that very day. And in Romans 5:12, Paul confirmed this fact, and told us, "Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned."
Just as surely as the infusion of hormones changes a boy or a girl into adults, something happened inside of man that day for which there is no cure except faith in Jesus Christ for our redemption. Thereby, we receive a down payment on eternal life, the final payment of which will be made on the day of the resurrection.
If there had been other couples created, death could not have come upon all men through Adam. Apparently, it means that Eve was the mother of all except Adam and Eve, whom God created. We have no hint as to how long Eve lived, nor do we know how many children she bore. We do see however, from this verse, that Cain married his sister, which was still common two millennia later, when Abram married his half sister, the daughter of his father (Gen. 20:12).
To the question of how many children did Eve have, I would have to add the question of how many Adam had as well. One thing we can readily see is that she bore enough to get the first "begotten" generation well under way. I will assume that she was called "granny" by at least three or four generations, but I would believe even more.
We are not told specifically whether Cain or Abel were married, nor whether they had children, but in Genesis 4:17, when Cain went out from the presence of the Lord, we are told that he "knew" his wife and he bare Enoch. The implication here is strong that he was already married. Perhaps he also had children whom he was forced to leave behind. It is very likely that Abel had children, as well. The fact that we are told that Cain was a farmer and Abel was a keeper of flocks implies a pretty well developed society with specialization of occupation, and that Cain and Abel were the two oldest people there besides the Mother and Father of them all.
Concerning Adam, we are told nothing about how many children he had begotten before the martyrdom of Abel. However, we are told, in Genesis 5:1-5, that Adam was 130 years of age when he begat Seth: "And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters: and all the days that Adam lived were nine hundred and thirty years: and he died."
We are also told something about Seth which should give us a clue to the diversity of the people who descended from Adam and Eve. It says in Gen. 5:3, "Adam begat Seth in his own likeness, after his image." This statement strongly implies that there was some kind of physical difference between him and all of the children of Adam and Eve unto that time. Consider the variety of appearances of the peoples of the Earth today. It must have been something like that.
It us unreasonable to believe that Adam and Eve had no more than two children before they were 130 years of age, and those all bearing years. That is especially so in view of the fact that the second commandment on God's list for them was that they should "Be fruitful, and multiply, and replenish the earth, and subdue it."