The discription for producing Mana in a city (like producing growth or research) says something like "Produces 1 Mana per essence". I tried that out with Pariden in my capital, which was a conclave. The city had four essence (two from the beginning, one from the building only Pariden can build, one from Oracle city expansion) but still producing Mana only added 1 Mana per turn in total instead of 4.
