I have an IMac purchased in 2011. Do I need to upgrade to 15.6.1 ?

The model year of an iMac is available at Apple menu > About This Mac.

An early 2009 iMac can be upgraded to El Capitan 10.11.6 but no later version. See: How to get old versions of macOS - Apple Support

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to get old versions of macOS - Apple Support

A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. See: How to get old versions of macOS - Apple Support

A mid 2014 and late 2014 can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to get old versions of macOS - Apple Support

A late 2015 or later can be upgraded to the current version of Monterey macOS 12. See: How to upgrade to macOS Monterey - Apple Support

15.6.1 is the operating system version for iPhone and iPad it is not for Mac.

The Mac got macOS 12.5.1, and no, your Mac model cannot update to it.