I used an image with a tangential section on the growing end of the protein found in some alveolar type II cells, where clearly the ribosomes were aligned (not in spirals) in a grid.  Using the pretty standard measurement of 100 nm for the distance between major dense bands in the periodicity found in these protein aggregations within the RER, it seems that in a complete period, dense to dense band there are four ribosomes (4 ribosomes per 100 nm) and if one calculates the continuous stacking of these bands (rather than having each period bound individually by RER membranes) then it becomes 3 ribosomes.  The number of ribosomes found in routine sections along the leading edge of protein production is approximately 1.7/period (when counting hundreds of sites, and two species of mammal). The count just in this particular image along the leading edge is @1.6 ribosomes, while measured from the tangential grid of ribosomes, it is 4 per 100 nm.

Images from ferret alveolar type II cells give a little more information about the ribosomal organization, as they are more regular and banding is more easily seen, profiles are more orderly, whereas images from guinea pig intracisternal bodies are more often curved, with the banding crossing, or being circular and disorderly, and also there are larger composites of banding patterns. In mongrel dogs… there is typically a single period with 7 bands…. (in some sense the dog intracisternal bodies resemble a little bit… the birbeck granules in Langerhans cells).  Here is the image used for these numbers.

(red lines are @ 100 nm, centered over contiguous ribosomes, the 100 nm length was calculated from the mean of the proximate banding patterns, shown in dotted lines)