Amazon's distribution/fulfillment centers exceed the combined number of distribution centers within retail. Please note that a number of the big box retailers are missing but the numbers don't lie. Amazon has nearly 2x more than the entire retail industry. Some may argue this is excessive whilst others may say this sets up Amazon for the next several decades of multi-channel fulfillment.
When taking stores into consideration as fulfillment centers, Amazon falls to the middle. With that said, this is a bit of a stretch in comparison. The economics and throughput of stores pales in comparison to distribution centers. The scale, reach and automated nature of Amazon fulfillment centers clearly sets Amazon up to leverage distribution as a differentiator for decades to come.