What can I upgrade my iMac 27 inch Mid 2011 to?

Then 10.13.6 is as far as it (2011 iMac) will go, you simply cannot go any further with a 2011. If you want or need to go to newer versions of Mac OS then you need to buy a newer machine. If you can afford a new one the new M1 based 24" iMacs are great and the 2020 27" iMacs are great. Even the Mac mini is a huge upgrade over your 2011 and you can get a very nice one for less than $1000, all you need is a display (about another $100) and you have a brand new Mac.

If you cannot get a new one, then consider Apple's refurbished iMacs which are identical to new, have the same warranty as new, are eligible for AppleCare, and can save hundreds of $$. However only buy one with a minimum of 16GB of RAM and largest SSD you can afford. Do not buy a Fusion Drive or any HD based Mac.

Still too rich for your blood consider a used Mac but buy it from a reputable source such as www.macsales.com. DO NOT buy from E-Bay, Amazon or your local computer shop. Macsales specializes in Macs, guarantees them and you can even upgrade them with Macsales RAM, SSD's etc.

Best of luck.

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

A late-2009 through mid-2011 iMac can be upgraded to High Sierra 10.13.6 but no later version.

A late-2012 through late-2013 iMac can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683

A mid-2014 or later can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT201475

without knowing the model year of the iMac we cannot answer your question, please let us know what you have.

Late 2009 - 2011 iMac will go to 10.13.6

2012 - will go to 10.14.x

2013 to 10.15.7

2014 to 2021 to 11.5.2

If you have older equipment that is limited, you should strongly consider replacing the machine. However remember that 11.5.x will only run 64 bit apps, so if you have older legacy third party apps that are only 32 bit apps they will not run on 11.0 and later so you will either need to upgrade or replace them. If you are unsure what they are navigate to roaringapps.com and check.