What is a good amount of people interviewed for idea validation?

I have an mobile app idea in the messaging space. I know the target audience. I just need to find places where I can interview them. But before that, I want to set a goal for myself for the number of interviews for my idea validation. I know that the more people I talk to the better, but realistically, in a week, how many people should I target to receive feedback on the idea? In your experience, what's been a "good enough" number?