It's an every year thing like kers says. It's also the first OS to support the new Apple CPU's and will apply more as the new hardware comes out this fall.
The current status is:
Mojave - 10.14 - runs 32bit & 64bit apps
Catalina - 10.15 - runs 64bit apps only
Big Sur - 11.0 - public beta this fall, supports new processors
Don't plan to move to it this year, maybe next year. Apple has been good about doing security updates for multiple platforms. I'm still on 10.14, and will be there for a while.