Thanks to an insider, I was sent a bit of information that provides a little more insight about the rings that Microsoft has for the entire flighting process and it’s much smaller than what you would expect for a public program.
Let’s start all the way at the development stage which consists of the Xbox Validation ring, this is limited to 50 consoles, and is likely the ring used by those developing features for the Xbox. Following this group is the internal Dogfood ring that has 200 consoles in it, followed by the Xbox TakeHome ring with 1,000 consoles and then finally for the internal rings is MS Beta which consists of 5,000 consoles.
The Public Preview ring has 260,000 consoles but the information I have does not break this down into the small sub-rings (Alpha, Beta, Ring 3, or Ring 4), so I don’t know if this is the entire population of Insiders and it is possible this information is using the older preview program stats. If this figure is accurate, it explains why it is difficult to get into the program as we know that there have been tens of millions consoles sold and over 50 million Xbox Live users.