In a recent Matt Cutts video, where the head of Google’s search spam team answered the question “Does linking my two sites together violate the quality guidelines?”
The short answer is no; but, there is always a but. If you link hundreds of unrelated sites together, then it would violate Google’s guidelines.
Matt Cutts explained that if you had a few sites, related, part of your network of sites, you can link them together. That is, especially if they are related and useful to each other. But when you start linking thousands of sites together and they are unrelated, then that would be a violation of Google’s guidelines.
What is the magic number? How many sites are too many sites? Well, Matt Cutts did say in the video that 50, or 80 or a 150 sites may start looking a bit over the top. He said, “but you have fifty sites or eighty sites or a hundred and fifty sites, and then suddenly linking all of those sites starts to look a lot more like a link network.”
Here is the video: