While some understand the reference to Adam in Genesis to be a general reference to mankind as a whole or the creation of more than one couple, most conservative scholars reject such a view and understand the Genesis account to refer to the creation of a literal Adam and Eve as a single couple. This is further supported by the NT. For instance Paul understood the OT to refer to a literal Adam and Eve (see Rom. 5:14; 2 Cor. 11:3; 1 Tim. 2:12-13). He clearly understood the reference to Adam and Eve to the first man and woman.

As to incest, it was not considered a sin and was not prohibited for Adam and early man. If the race was to populate and fulfill the command of Gen. 1:28, there is little doubt that Adam’s sons and daughters had to have married their own sisters and brothers if the race was to populate the earth, but due to the purity of the race as evidenced also by the long length of life, there were no adverse effects as we see happening today. Gradually, as the effects of sin took its toll on the human race, marrying one’s own sister, etc., began to create hereditary problems.

Here is Ryrie’s comment on this issue from his book Basic Theology (1986 ed) which I would highly recommend.