What is the latest macOS for my iMac Retina 5k 2015?

Apple introduced two 27" 5K Retina iMacs in 2015.

Big Sur is the end of the line for the Mid 2015 one. Monterey is the end of the line for the Late 2015 one. If you look in the About This Mac dialog in the Apple () menu, it should tell you which of these two 27" iMacs you have.

How to download and install macOS - Apple Support

Upgrading a macOS depends upon the model Mac, specifications, and year. To get more information about your computer, choose:  (Apple menu) > 'About This Mac' in the upper left corner of any window, then "More Info..." or "System Report". There's more about this in "About System Information [Profiler] on your Mac" - System Information User Guide for Mac - Apple Support

Use the computer information, including the model year, and the links below to determine what is possible with your computer.

Click on this link to read how to get different versions of macOS --> How to download and install macOS - Apple Support

- The article also has links to system requirements for each OS version which you should check against your exact computer model. For any model produced in the last 5 years or so you should use the system requirements links for each OS version instead. This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility