Graham, many years ago I asked our developing technicians in the lab that I worked in, this same question.
Their findings were that approximately 8 rolls per litre of 35mm 36 frames was the closest. One could stretch this by about 20%, meaning 10 rolls per litre. They added caveats like, the actual amount of active ingredients per litre etc, etc,.
I also remember reading, (15 or so years ago) on either an Agfa or Kodak kit, 8 rolls of 35mm 36 frame film per litre.
I myself use Jobo rotary and develop 4 rolls at a time in 500ml (35mm).