"So many" is a relative amount. I was always under the impression that most of Earth's population was no longer alive. The man and his son do come across people with fair regularity, and that might give readers reason to think that a lot of people survived; however, with resources being so finite, it makes sense that most survivors would be traveling to and from the same limited number of locations. The text doesn't tell readers what the cataclysmic ending to society was caused by, nor does the text give readers a percentage of survivors. If the disaster was nuclear, then I think the survivors were probably people that lived far enough away from the blast zones and following radiation fallout to have survived. A nuclear disaster does make sense based on how the man immediately fills the bathtub with fresh water after seeing and feeling a “long shear of light and series of concussions.” I suppose that could be describing a meteor crashing into Earth, but surviving that would be similar to a nuclear holocaust minus the radiation. If the disaster was biological, then the diversity of genetics would cause natural selection to kick in and award certain people that had specific adaptations in their immune system the ability survive.

We don’t really know why so many people survived the “apocalypse” in Cormac McCarthy’s The Road. I’m not really sure I would say “so many” survived. The father and son walk for great stretches without encountering anyone. However, it’s true that they are continually on the lookout, knowing that they could wander into a dangerous situation (anything that involves other people) at any time.

The reason we don’t know why so many people survived is because the author never tells us about the nature of the disaster. We simply don’t know what happened. McCarthy wanted us to focus on the aftermath of the disaster, not the cause of it.