The Census reports there are about 40k E-commerce companies in the US, but Shopify has over 100k stores, and it's just one of many E-commerce platforms. Internet Retailer does a great job providing data on the top 1000, but there's no one filling in below those top 1000. Clearly we have some bad reporting on numbers. RJ Metrics takes on trying to estimate the number of E-commerce companies and segment them by size of their revenue base. This is as good an effort as I've seen.