Adam and Eve had at least five sons and two daughters. See Genesis chapters four and five. The only explanation of the growth of humanity has to be that Adam and Eve's children sometimes married each other, since there was no law from God about incest yet.
There was also no birth control. As more and more babies were born, it was no longer necessary to marry a brother or a sister. This is a simple answer. You will find more information if you start reading from Genesis 1 through Genesis 5.
Adam and Eve could have had 500 or more children during their long lifespans.