I roughly sketched my own model on an office whiteboard and ended up with a subscription number just over 70 million, likely the result of weighting Amazon penetration in Asia more heavily. I also think it might not be generous enough to assume that 90 percent of retail subscription revenue comes from Prime. Somewhat outdated estimates of Amazon music subscriptions indicate that only a few million users have subscribed to the service, which, ranging from $48 to $180 per year, doesn’t quite account for 10 percent of sales — though things get more complicated when you add in other subscription services, so it’s hard to say for sure.

https://techcrunch.com/2017/02/13/amazon-hints-at-prime-sales-in-latest-filing/