At a technical level, every species on earth had its first male and female. Ground zero, so to speak. The first pair of breeding members of their species. In most instances those "originals" can breed with the species from which they evolved, but not until the new species begins breeding only within itself does the genetic code become fixed and the new species established (though future mutations could produce another new potential species or simply variations of the same one... like skin color.
What makes story of Adam and Eve interesting is its recognition of the original "breeding pair" that established humans.
That they had multiple children beyond Cain and Abel is likely a given, since it was pretty rare in those days for humans without birth control to simply stop having sex. How many children is pure speculation, but clearly enough to establish humans as a distinct species.
Birth defects among related couples are as rare as among related couples. Generations of interbreeding does increase the chance of a defective mutated gene being in the offspring, but first you need the mutation to occur and it may never do so. So, your "science" is pretty weak in this area, if neither "Adam or Eve" had a defective gene, then there would have been zero offspring with a birth defects unless a new mutation occurred during gestation, which is not more common among related couples.
https://theconversation.com/whats-the-genetic-disease-risk-for-children-of-related-couples-7599#:~:text=Marriages%20between%20people%20who%20are%20related%20is%20more,is%20almost%20as%20high%20as%20any%20other%20couple.
All that said, I think people that argue the Bible is science or history are off base. The Bible is a theology book with a lot of science and history in it. Its texts has been simplified in many instances (how else would you summarize something as major as the creation of humans in a few chapters in a way that could be passed down through oral traditions) with the emphasis on the message and not the details.
Finally, numbers in religious texts have spiritual meaning and someone of the time would read it differently than we do. Numbers like 7 and 40 are repeated over and over again in the Bible. 800 years represents "a long time" shows up in many Middle Eastern, Indian, and Egyptian texts. So, what we know is the original breeding pair of humans lived a long time and had many children. Those children formed the basis for the human species.
The same is likely true of all new species, since for the species to establish itself the first alpha members must survive long enough and produce enough offspring to cement the new species existence.
