Learn English – What does the phrase “You don’t say” represent

This construction is usually (or often, anyway) used in team situations:

There were more of us than there were of them.

"More of XX" meaning more people on the team. However, it is also used (perhaps ironically/humourously) when one team has only a single person. In that case, there are always more of them than of him/her.

When expressing frustration or exasperation, we often begin by saying something like, "If I had only known..."

• If I had known your plane was going to be two hours late, I wouldn't have rushed to the airport.
• If I had known you were going to get such bad grades on your report card, I wouldn't have let you go to the dance last weekend.
• Had I known the van was going to overheat, I would have driven the car instead.

Yet, in all these instances, life offers no crystal ball; there's no way of knowing what the future holds, or to be cognizant of something we are unaware of. This is the essence of "You don't know what you don't know," only it's being expressed in a comical way, much like Yogi Berra might have said.

(To those unfamiliar with the Yogi Berra reference, he was a professional baseball player who was reknowned for his wry way of saying things, such as, "It ain't over 'til it's over," and, "You can observe a lot by watching." That said, you couldn't know what you didn't know, but now you know.)

Generally speaking, it would not be a good way to express something in formal writing, unless perhaps you were deliberately injecting humor.