What is the latest version of Photos?

The latest version of Photos is always the one with the latest version of the OS, as Photos is now installed and updated as part of the OS.

So, if you have the latest OS you have the latest Photos. So, Mac OS 11.4 gives you the most recent update to Photos v6

And on your system, as shown below your question, you will have Photos 2. Photos gas changed a lot since then. Particularly the Cloud features and the face recognition have changed, Why are you asking? which features are you missing?