Everything is happening at the same time. Or at least, that's how she can now perceive time. She knows the number because at some point in the future it's given to her.
So many stories, so little time.

That is the part that actually unlocks the mystery and by no means a mistake.
Your title is Time "Loop" as being non linear the time dimension is an actual loop as the past effects the future and the future effects the past.
That's why Louise knows she has to go forward with her life knowing its fatal outcomeas if she altered it would alter the loop and may have resulted in China starting a Nuclear war.