Thank you, Waltern.
Is this expression used much often? Like: you often come across a certain store, but you hardly ever buy something from it. Then, one day, you need to get something from this store, but you can't find one(you don't know the address). Could I say something like : "the one time I need this store"....? Or would it sound odd to you?
You need to understand this is used somewhat figuratively. He could say the same thing again in two weeks. He's not really counting the times. He just means he really needs her and wishes she was there. (There were many other times he didn't need her as much and she was there.) So "the one time" is probably a bit of an exaggeration.
Can you imagine in a business they only did something important one time?