Amazon Music surpasses 55 million users, "nearly all" of which are paying subscribers Closing in on Apple and Spotify January 22, 2020 https://www.techspot.com/news/83664-amazon-music-surpasses-55-million-users-nearly-all.html
I used Amazon Music Unlimited for a few months. I used 30 day trials, for both of my Amazon accounts and then used it for another month after I forgot to cancel one of the trial subscriptions. It's not something I want pay extra for. Even though the music library included with the standard Prime Music is very limited, Spotify and Pandora, both offer a very large music library free of charge.
You can get Amazon Music Unlimited free for three months April 9, 2020 https://www.neowin.net/news/you-can-get-amazon-music-unlimited-free-for-three-months