You can expect only a small percentage of people who click to subscribe. A sample size of 6 is too small to draw any conclusions. I would say that you have not paid enough money for advertising to know if it works or not.
But the reason people say to use your network isn’t really for finding customers - it’s to get feedback from potential customers. That is way more valuable than any money they might give you. It’s pretty uncommon to have instant product-market fit. A lot of startups have to make significant changes to the product or pivot entirely to become successful.
One of the things I learned from real customers is that not everyone uses a computer the same way I do, and things that I assumed everyone did are not as common as I thought. It changed how I design software and such a lesson is difficult to learn when you’re just in your own bubble.