Let's assume you're writing your own copy (maybe you can't afford to hire a professional, or you just love writing copy for your own products).
One of your first tasks as a copywriter is to break down the skepticism that your prospects (readers, site visitors, subscribers) are feeling, and get them to believe you – even just a little bit.
Once that initial barrier of skepticism comes down, you have a chance of making a sale.
How do you break through that skepticism?
Testimonials.
You need lots of testimonials in your copy. How many? As many as you can get.
Here's a good rule of thumb: however many you have now, get 25% more.