If the goal is to get to know your customers, you will want to talk to around 5 that fit each customer segment / user type / persona. You want to get to a point where you can describe that persona as if they were a friend of yours and you could naturally explain who they are and why they use your product.
The less familiar you are with your product or the more complex it is, the more interviews you are probably going to need. For instance, you may need closer to 10 to reach the point that you feel like you get who they who are and what they’re doing with your product.
More commonly though, your goal is probably to inform a product change. In that case, I recommend starting with 3-5 interviews to answer a specific question. Once you start seeing a pattern you can stop interviewing.
Most likely you’ll start seeing some common themes after 3 and by the time you reach 4 or 5 you realize you’re hearing the same thing over and over. That’s when continuing doesn’t provide any additional insights so you can stop.