ANSWER: Some people believe Adam and Eve were just two representative people among many, but that is not what the Bible says. Scripture reveals only TWO people were specially and personally created by the hand of God. Additionally, evidence to support the fact that the Lord created only two original humans can be found in the meaning of Eve's name.
After Eve was created she was brought to Adam. Upon meeting his helpmate for the first time he named her in the Hebrew ishshah, or "woman" (Strong's Concordance #H802), which is the generic designation of all females (Genesis 2:23). Interestingly, it was only AFTER they sinned that the first man gave his wife a personal name (Genesis 3:20).
Eve's name in Genesis 3:20 is translated from the Hebrew word chavvah (Strong's #H2332) which literally means "life" or "living." Inserting the actual Hebrew meaning of her name into the verse in question, we get the following, "And Adam called his wife's name LIFE, because she was the mother of ALL LIVING."
Eve was named LIFE, or the mother of ALL living, because out of her would spring the ENTIRE (not just some of or most of) human race! Her name does not leave any room for human beings to be created or produced by any other woman or means.
Now let us answer your second question regarding how the human family could have gotten progressively bigger if only two humans were originally created. The Bible only mentions, by name, three children produced by our first parents. Cain was the firstborn child (Genesis 4:1) followed by Abel (verse 2). Seth came into the world sometime after Cain killed Abel, when Adam was one hundred and thirty years old (Genesis 4:25). Genesis 5 tells us that after Seth was born other children were brought into the world.
After he begot Seth, the days of Adam were eight hundred years; and he had SONS and DAUGHTERS (Genesis 5:4)
Adam lived a total of nine hundred and thirty (930) years! It would be safe to assume that Eve lived at least as long, maybe even longer, than her husband. Even if we assume they only had three children in their first 130 years together, they had PLENTY of time after this to take full advantage of God's blessing and obey His command to procreate.
28. And God blessed them. And God said to them, "Be fruitful and multiply, and replenish the earth, and subdue it; and have dominion over the fish of the sea and over the fowl of heaven and over every living thing that moves upon the earth" (Genesis 1:28, HBFV)
A footnote in the writings of the Jewish historian Flavius Josephus (Antiquities of the Jews, Book 1, Chapter 2) stated that a very old tradition said the first couple had a total of THIRTY-THREE (33) sons and TWENTY-ONE (21) daughters! Some mathematicians have calculated that at the time of Noah's Flood, which was roughly 1,600 years after the creation of man, that the population on earth could conservatively been at least ONE BILLION people!
Additionally, since the first couple were created perfect, their genetic makeup would be perfect and so would that of their first few children. With no genetic problems to enter the picture, God allowed them (for a time) to marry each other. The world's population grew very quickly. As time went by, however, genetic imperfections would make close-kin marriages impractical and a real risk not only for the mother but also the children who could more easily be born with genetic defects and handicaps.
The Bible record leads us to conclude that God personally created only two humans, Adam and Eve, and then let the biological processes and urges He placed in them do the rest to populate the earth.