Historically, Apple has maintained the 3 most recent versions of macOS. The current release that may get functionality enhancements as well as security updates.
And the previous 2 macOS versions which get security updates, and on rare occasions enough functionally to allow that macOS to work with another Apple product (such as an iPhone or iPad).
Catalina 10.15 is the current release
Mojave 10.14 and High Sierra 10.13 are the other 2 releases that will be getting security updates.
Sometime around the Fall 2020, a new macOS 10.16 will be released and Mojave will be the oldest supported release.
Then in the Fall of 2021 macOS 10.17 should be released, and Mojave will no longer get security updates. That is your target date for figuring out what you are going to do. Stay with Mojave, or switch to a newer supported release.
All of this is based on 15 years of historical behavior. Apple could change the model they use at any time, so your mileage may vary.