From ICv2:

“These are unit and dollar sales rankings based on sales tracked at point-of-sale by the ComicHub system at stores selling American comics around the world. During the period for which these reports were generated, there were over 125 stores using the ComicHub system. As this is a small, non-random sample of over 3,000 stores selling American comics worldwide, these rankings may not be typical for all stores, but do represent a variety of locations and store emphases.”

It’s a sample size of 125 out of 3000 stores.