If I have macOS Sierra version 10.12.6 what should I be updating to

What is your Mac’s model year and type? You Will need to see your mac type to determine where you can update to.

Go to Apple logo in menu bar>about this mac to check this.

How far you can go will depend on exactly what model you have.

All Macs capable of running 10.12 Sierra are capable of running 10.13 High Sierra.

Beyond that, it will depend on the model year of your Mac.