What is the latest IOS I can have on an iMac Retina 5K, 27inch, Late 2015 (4GHz intel core i7 / 8GB 1867 MHz DDR3)

Hello All,

I am back from a long period overseas and I would like to update/upgrade my iMac Retina 5K, 27inch, Late 2015 (4GHz intel core i7 / 8GB 1867 MHz DDR3) with latest possible IOS.

It's currently running on Sierra 10.12.6. It's nearly new and I really would like to keep it for longer.

Thanks a lot