Before their respective releases, Windows 7 went through two public previews while Windows 8 went through three. Both were quite the achievement at the time: After all, Microsoft was showing off its most important piece of software to the world before it was finished.
During the Windows Insider Program, Microsoft released 15 public preview builds of Windows 10 for PCs (Windows 10 Mobile is coming later). Here they are in order: 9841, 9860, 9879, 9926, 10041, 10049, 10061, 10074, 10122, 10130,10158, 10159, 10162, 10166, and 10240.
Windows 10 builds were made available in private and public rings. The private ones included: Canary Ring (daily updates and only accessible by Windows developers), Operating Systems Group Ring (after the Canary Ring approves a build), and Microsoft Ring (after the OSG Ring approves a build, Microsoft employees can test it internally). The 15 public builds went through two Windows Insider Rings: Fast (builds that have been approved by the Microsoft Ring) and Slow (builds that have no major issues in the Fast Ring).
One of the big reasons it was even technically possible to deliver so many builds is because of the changes the Windows 10 team made to the build upgrading process. To be clear, the core upgrade mechanism was not new. This is the same in-place upgrade technology that is already available in Windows 8 and Windows 8.1 (ESD files have been enhanced, but they’re still largely the same).
I learned that there were multiple new components, though, including targeting, pool management, registration, the insider channel, and so on. The most important new part is that the Windows 10 team was (and is still) able to offer a specific group of people a given set of builds, letting them do an in-place upgrade when a new build became available.
Source: venturebeat