Try however many you need and share with us how they work. My opinion is you only need one if the slide is broken or the diaphragm is ripped.
Even then you can use JB Weld to patch a slide, or replace a torn diaphragm with an aftermarket diaphragm if the slide is good. Search on here for methods of diaphragm replacement. yamaha sells the diaphragm and slide as a single unit, but you can but combo pieces like the named vendor offers, or individual diaphragms to be attached to a good OEM slide. Be careful, as the slides are brittle.