Subscribers can collect a total of 48 free engrams over the course of the year. The partnership suggests that the game will eventually be making its way to Amazon's game streaming service much like how it's already playable on Google Stadia.
https://www.notebookcheck.net/Destiny-2-is-handing-out-exotic-weapons-and-engrams-to-Amazon-Prime-members-for-the-next-year-because-why-not.506132.0.html