It's an incredible number to reach, and it's important to highlight that they are not all licensed for free use in commercial work, nor are they all of suitable resolution to use as stock images. Looking at flickr creative commons  I can see that around 17.5 million images have a licence that would let anyone use them as stock (with attribution of course), the remainder have non-commercial licenses.
Remember that number is just on one site, the internet has millions more creative commons images, and unlike flickr which is predominantly amateur snapshots, some of the CC  images available are produced specifically for use as stock photos. Currently they are hard for end users to find reliably, but as search engines develop that will not always be the case.
Why am I writing about this? It's relevant to my recent post about giving away stock photos for free , and also because it is news to me; I thought that it was only a tiny minority of images on flickr that were CC licensed. When istock got to 1 million a few years ago I thought it was a mind boggling number of stock images and their collection must be almost universal! News like this puts into perspective how small the stock industry is compared to the consumer photography market.