Q: How many online retailers are there worldwide?

A: At least 30 million, counting everyone who sells on eBay and China's Taobao.

We get this question all the time. We've never seen a reliable estimate.
But here's some data that might help. There are 25 million sellers on eBay worldwide and 6 million on Taobao, China's equivalent of eBay. There's probably little overlap between the two, so that's at least 30 million sellers. Now some of them are individuals who don't make a living selling online, so the number of SMBs is probably more like 25 million.
Amazon says 2 million companies sell on its marketplace, but the overlap with eBay is probably high. So I wouldn't bother adding any to the 25 million number.
There are, of course, many companies that sell online but not on eBay, Amazon or Taobao. That would linclude the largest retailers in the world, such as Wal-Mart, Tesco, Carrefour, Apple, Dell, and so forth. The number of small-to-midsized businesses selling online but not on the major marketplaces I would guess is under 1 million.
All in all, if someone asked me for an estimate of the number of small or midsized businesses actively selling online today I would say 25 million worldwide.