How much? That is a challenging marketing question. Given enough resources, we can use some precise mathematical models to figure this out. But many times, we just do not have those resources to conduct extensive quantitative analysis.
Enter magic numbers. By asking questions of many different groups of people, patterns emerge. And repeat. And those are the magic numbers. Pay attention to them and you can make sound decisions about demand and pricing for your product or service. As you get moving into the market they can always be fine tuned. But many times it is better to get into the game imprecisely than precisely miss the market window.