It all depends on how much you are willing to spend. If you are DIY guy and can change the injector yourself, you can get away with just one. If you depend on the mechanic to change the injectors, it makes sense to have them change all of them at the same time.
Changing them all at once makes sense with the high mileage. With that said I have seen original injectors go up to 300k miles. It depends a many factors inclduing maintenace and diesel used in your area.