I was looking at this today. My 1941 Schwinn's ND Model D came with 23 discs. Similar to what's called out in this dwg:
But clearly this changed over time. Dropping to 21 discs called out in step 4 of this dwg:
And down to the now default number of 17 discs per set (D278).
The Schwinn Repair Manual that
@rustjunkie provided here has a great section on coaster brakes. This page has some info on the disc sets, and the note at the bottom in Step 7 pretty much sums it all up:
Having a higher number of thinner discs would give the brake slightly more braking surface area rubbing together but I doubt it makes much of a difference if you can lock the brakes up with 17 thicker discs.
I also thought this note at the bottom of step 4 on assembling the disc sets was interesting and have never heard this before. I thought you HAD TO start and end with Steel discs:
The other thing that I've been following, is the many methods of lubricating the ND coaster brake. I hadn't noticed that the recommendations from ND changed over time. It has always been grease on the bearings and oil on the discs, but I imagine as people realized that light motor oil was too thin and could work its way out through the grease, they started to increase the weight of the oil. Thicker oil sticks to the discs better and doesn't mix with the bearing grease and work its way out as easy.
Old 20 weight oil call out:
Here they are calling out Hypoid (gear) Oil:
And the Schwinn Repair Manual says Hypoid or Castor Oil:
I hope this post can help others unravel the differences in number of discs seen in these hubs and how to lubricate them correctly. Fast forwarding to the Schwinn Repair Manual advice is probably the best IMHO.
3/4" total disc stack height and thick gear oil on the discs