My two European River Cruises were in 2006 and 2010. At that time, the ship capacities were in the range of 125. For three PDs, that made each "flock" about 42. Now I see that the smallest ships have a capacity of 140. For three PDs, that makes 46 tourists per PD. Adding a fourth PD would make the number per PD only 35. I also noticed that some ships are listed as having a passenger capacity of 162. I didn't find what cruises used the larger ships, but, I would assume, those ships employ four PDs.