As far as I’m aware, it is not possible to get personal details from a download. And that’s how it should be, for privacy reasons. If your friends are willing to share how many times they downloaded your game, you can ask them, and subtract that number from the 20 downloads.
There is an exception with users who have paid for your project. You can see their e-mail, but not how many times they’ve downloaded your project.
Hope that helps.