Every time someone listens to a song Spotify, it generates royalties. On average, Spotify reports that it pays out between $0.006 and $0.0084 per stream to rights holders. Unfortunately, there is no clear cut rate, especially when that royalty is divided among multiple rights holders.
The rates paid to publishers and writers will depend on multiple factors, and will fluctuate over time. There are also additional factors that will affect rates, such as what country the stream occurred in. In addition, traditionally a use would only generate one type of royalty, but streams will generate both a mechanical and performance royalty.
In 2017, it was reported that it took about 700,000 ad supported streams to generate $100 in mechanical royalties and 150,000 premium tier streams to generate $100 in mechanical income.
Spotify pays out 70% of their revenue to the master and publishing owners. So, as their user base grows, their rates may grow.