Though there are two English words, “back” and “number,” this is not an English expression. If you use it while watching sports, an English native speaker might understand the meaning, but to be clear, you’ll need a new term.
The key here is that the number is not actually written on a person’s back, it’s on their uniform, usually a jersey. So, you can say, “jersey number” or “uniform number.” The most common thing to say though is just “number.” You can also sometimes hear “player number.”
“She is number 13,” or, “His number is 21,” are both common ways to express 백넘버 in English.