how far can i upgrade os for 2009 imac?

There was an Early 2009 iMac and a Late 2009 iMac.

see > Identify your iMac - Apple Support

The Early 2009 can be upgraded to 10.11 El Capitan

see > How to download OS X El Capitan - Apple Support

The Late 2009 can be upgraded to the current macOS 10.13 High Sierra

see > macOS - How to Upgrade - Apple

Yes, it can get the latest 10.13.1 High Sierra

Late 2009 iMac is supported.

How to download macOS High Sierra - Apple Support