After Sierra, you have High Sierra, Mojave and now Catalina (I DO NOT ADVISE UPDATING TO CATALINA YET IF YOU CAN AVOID IT).
High Sierra is stopping its user support (current updates) this coming November 2020.
Sierra:
Support Ending November 30, 2019
In keeping with Apple's release cycle, macOS 10.12 Sierra will no longer be receiving security updates. Sierra was replaced by High Sierra 10.13, Mojave 10.14, and the newest Catalina 10.15. Our latest fully-supported operating system if macOS Mojave (10.14).
High Sierra:
"Support Ending November 30, 2020
In keeping with Apple's release cycle, macOS 10.13 High Sierra will no longer be receiving security updates as of November 2020. High Sierra was replaced by 10.14 Mojave, and the latest version,10.15 Catalina."