Plasma Productions

Seth was not the sixth person on Earth.  First came Adam, then Eve.  Then they had two sons - Cain and Abel.  That makes four people so far - Adam and Eve, and their two sons.  Cain killed Abel, but then Genesis 4:25 tells us, "Adam lay with his wife again, and she gave birth to a son and named him Seth, saying, 'God has granted me another child in place of Abel, since Cain killed him.'"  Seth replaced Abel, so Seth was Adam and Eve's third son, and the fifth person on Earth (after Adam, Eve, Cain, and Abel).  Then the Bible tells us that Adam and Eve had "other sons and daughters," and those sons and daughters had other sons and daughters, and so on until the Earth was populated.