The question is which is better from the efficiency point of view: One big propeller or multiple smaller ones? Or are they the same? My general impression is that a bigger one will have much more better efficiency as if you consider the following points: 1. lift is proportional to the area of the disk (momentum theory); 2. biggest loses are the tip loses, more propellers, more tips. How all this changes with shrouded propellers? You eliminate tip loses, but is the assumption that increasing the number of propellers increase the shrouded wetted area and increases losses as well?