what is the latest macOS my iMac supports?

Hello,

You are currently running the latest version of macOS your iMac (mid 2011) can support: macOS High Sierra - Technical Specifications

macOS Mojave requires a late 2012 or newer iMac: macOS Mojave - Technical Specifications

Actually your computer is 12 years old and is considered quite old. I would recommend considering replacing it as it cannot run current versions of Mac OS, current versions of third party apps and the internal components can fail at any point. Typically, most users replace their computers every 6-10 years, you are overdue.

You've hit the limit.

10.13.6 is as far as it can go.