The Rule Of Two's in Life After 60

My dad died when I was 12. He was a Navy Seabee and was stationed in Honolulu Hawaii on Dec 7th, 1941. He helped clean up Pearl Harbor. Of the things he taught me in that short time, one became known as the rule of two’s.
Dad was always building something and when it came to nails, screws, and the such he taught me that if there was one, there was two. To this day I don’t know if dad set things up that way by always buying in even numbers or it was just one of those weird coincidences of the universe. But if dad sent me to get a bolt, he would always tell me, there are at least two in the drawer.
As I grew older I learned the exception to the rule. There are few absolutes. So what about that rule of two’s? I think dad instilled that rule in my mind so I would not give up.