Genesis 3:20 states that Adam named his wife Eve "because she had to become the mother of everyone living." This would seem to pretty clearly state that they're the only humans at this point.
The other thing about the story of Cain and Abel is that it is told in a non-specific time period, "at the expiration of some time" as Genesis 4:3 says. So, unless I've missed something, it would be pretty clear that at the very least, Adam and Eve would've had more sons and daughters by the time Abel's death happened. It could even have been possible for Adam and Eve's other kids to have had sons and daughters themselves, but there's just no specific information to go on.
With that said, I'm not saying transhuman68 is wrong or anything, because regardless it does seem clear that they were living in a populated world by that time, whether it was because of Adam and Eve's kids or some other group of humans. Seth wasn't born until Adam was 130 years old. So Cain and Abel could've been the first kids but Abel's death could've happened 20-100 years from their birth, by which time there would've been many kids, but apparently the Bible doesn't bother to follow the other kids' history beyond Seth.
--sd-7