I have two versions of Photoshop CC that update on my desktop PC, v22 and v21. Auto update updates both. I don't know why I have two versions. I only try to use the latest v22. Is there a reason for this. Should I delete v21? Will deleting it affect v22?
You can have two versions, or you can uninstall one if you want. It can be useful when you upgrade to a new version to keep the previous version in case you find you have issues with the new version or if you find your plugins haven't been updated to the latest version of Photoshop.
IMO you shoul not remove Photoshop 2020. Photoshop 2021 has many known issues. Photoshop 2020 does not have Photoshop 2021 new features. However it is more reliable any has fewer issues then Photoshop 2021. Adobe has not finished development of Photoshop 2021. https://helpx.adobe.com/photoshop/kb/known-issues.html