According to Undead Labs, State of Decay has sold two million copies between the Steam and Xbox Live versions.
Official quote from Undead Labs:
"About this time last year, we had a million reasons to smile.
Now, we have two million reasons. That is, two million copies of the original State of Decay have been sold, between Steam and XBLA. We’re not even talking about DLC, here.
There just aren’t the right words to tell you how much this means to us. No marketing, no advertising, just you playing and streaming and spreading the word. You made this happen, and if we could individually thank each person who hit that download button, we would. Your enthusiasm and support have been everything. Thank you"