The Bible doesn't say specifically. However, thinking about it logically, I've always thought that Adam and Eve possibly had many other children. They lived for quite a long time, allowing for many generations of children. It may be that the Bible simply doesn't talk about these children because they weren't significant to the story.
Another thought would be that since humanity was more "pure" back then, there may have been some incest going on, since this would not lead to the types of abnormalities we would see today.
Either way allows for more women on Earth for Cain to make his wife. These also explain for there being enough people on Earth by the time Noah came around (just a few generations later) for it to be necessary for God to flood the entire Earth to wipe everyone out.
NOTE: I would like to reiterate that none of this is stated specifically in the Bible. These are just my educated guesses.