The figure is actually a tally of customers across several tiers that Amazon offers. Amazon Music Unlimited is the $9.99 a month plan that serves as a direct rival to Spotify and Apple Music. According to Amazon, subscriptions numbers on this tier grew by more than 50 percent last year alone.
The other tiers include: Amazon Music Unlimited ($3.99 single-device plan) for customers who just want to listen on an Echo speaker; Amazon Music for Prime subscribers, which includes ad-free access to over 2 million songs; ad-sponsored Amazon Music, a free plan offering access to top playlists and thousands of music stations; and Amazon HD, a high-definition tier costing $14.99 a month.
Amazon says its streaming service has grown nearly 50 percent year-on-year across the U.S., U.K., Germany, and Japan, and has more than doubled in its newer markets like France, Italy, Spain, and Mexico
Despite the respectable growth across its range of tiers, Amazon Music has some way to go before it catches up to Spotify, which in September announced it had 113 million paying customers.
https://www.macrumors.com/2020/01/23/amazon-music-has-over-55-million-subscribers/