welcome to the forum.
there’s not a straightforward way for regular users like us to do what you’re asking for (that I’m aware of).
you could get this information from the API using this as the base request (and iterating through all the pages needed to get all your observations): https://api.inaturalist.org/v1/observations?user_id=arthur_chapman&per_page=200
or you can use this to see the information in a more human-readable format:
https://jumear.github.io/stirfry/iNatAPIv1_observations.html?user_id=arthur_chapman&per_page=200&options=photourl
neither of these will automatically aggregate the total number of photos for you, but you could take what they give you and tally or sum things up on your own.
here’s a related discussion that may provide more information: https://forum.inaturalist.org/t/downloading-to-spreadsheet-with-multiple-image-paths/18971.