What's the latest OS compatible with MacBook Pro 2012?

macOS Catalina 10.15.7 (see: Identify your MacBook Pro model - Apple Support )

To upgrade, see instructions in How to get old versions of macOS - Apple Support

Also be aware that if you have any non-Apple 32-bit apps you will need to upgrade them if you upgrade to Catalina. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

10.15.7 Catalina for the 2012 13" Retina per Mactracker.