I think you're referring to the GitHub Engineering blog post [1] about our git storage tier. We [2] store your code on at least 3 servers, which is an improvement in many ways from our previous storage architecture. There are a lot of servers [3] powering things but not the millions it would require to give every customer three dedicated machines. Developing efficient solutions to problems is a requirement (and a fun challenge!) for anything at GitHub's scale.

[1] http://githubengineering.com/introducing-dgit/

[2] I'm a GitHubber. https://github.com/jssjr

[3] https://twitter.com/GitHubEng/status/730429227896463360