That’s like saying you have a limited amount of muscular endurance per day. It’s true, but it’s completely trainable. Meaning just like you can go from barely able to run a mile to being able to run an ultramarathon, you can do the same thing with your willpower. But unlike running a marathon, training your willpower will yield real, life improving tangible benefits assuming you can correctly figure out how to apply it for your benefit. Maybe a poor analogy because running an ultramarathon requires lots of willpower :)