What OS X is Apple's in-house business using?

I wonder what is the current OS X that Apple uses in-house as "reliable" to run their business?.

Does their in-house OS X change each year, or do they just rely on "old reliable" until they believe that the newer (whatever one) becomes the "old reliable"? Interesting to see what Apple uses currently.

Any thoughts? It would be scary if they upgrade all of their in-house systems to the latest OS X each year and risk all of the issues, especially with our data, apps purchases,...

What OS X is Apple's in-house business using?