[EricH]

Ether is right. If you want to optimize a smooth ride, lots of rollers are better (less "flat" distance between crowns of rollers). If you want to optimize manufacturability, fewer rollers are better (to a point--4 rollers to a wheel will most likely really, really make life miserable for the drivers). And it will depend on your design.

The center-hub designs typically carry about 6-8 rollers per wheel from what I've seen; the AM outer-hub design carries something like 32. I've seen an outer-hub design carry 8, and a combination of outer and inner carry something like 20. It varies depending on design and wheel designer.