As regards the children of Adam and Eve, the narrative of Cain suggests in at least three places that there were other people on Earth at his time:
First, in Genesis 4:14-15, he worries that he will encounter people who will want to kill him, and God gives him a mark of protection.
Second, he then moves to Nod, at which point he has a wife (Gen. 4:16-17). The wife is presumably not a child of Adam and Eve, as we do not hear of any other children being born after Abel until Seth, which is mentioned after six generations from Cain (Enoch, Irad, Mehujael, Methushael, Lamech, and Lamech's sons, Gen. 4:17-25). The birth of Seth comes when Adam is 130 years old, and daughters and other sons followed on after that (Gen. 5:3-4), while Cain is implied to be born very shortly after the Fall.
Third, Cain builds a city in Nod (Gen. 4:17), which assumes a community of some size already existing.
The only place in the creation narrative where it suggests only two humans were ever created is Genesis 3:20, "Now the man called his wife's name Eve, because she was the mother of all the living"—and this in itself does not look like a literal statement, as at that point she had not yet, in fact, given birth to anyone.
(Certainly it looks like only two humans were put in Eden; in the Elohist account, though, we have that male and female humans were created on the sixth day (Gen. 1:27) and their number is not specified, at least in the usual English translations.)