The following code snippet will show you how you can be notified via email when one of your downloads reaches a certain number of sales. We needed this recently when AffiliateWP reached its 500th sale. When this milestone was reached, we were able to instantly view the payment of the 500th sale and reward the customer for helping us reach our goal.
Copy and paste the code into your child theme’s functions.php or custom plugin. Change the download ID, and enter your sales milestone and you’re done.