If you had to pick a number I'd say 30 people at least. The more the better. And how many people do you need to call before you can determine this is a market problem? That's not a specific number but you want to know that it's widespread instead of concentrated on one person. A lot of people, especially people who don't like talking to people in traverse, they'll often latch on to an idea early because they don't want to talk to more people. They're too scared. But if you can notice that a problem is spread across 30 people that have no real connection to each other then there's a high chance that that is a market wide problem.