In a recent TechNet article itemizing new IT pro features for the Anniversary Update (codenamed "Redstone 1"), Microsoft officials included this line: "Windows 10, version 1607 is our third Windows 10 feature update released. Based on feedback from organizations moving to Windows 10, this will be our last feature update for 2016, with two additional feature updates expected in 2017."
The first Windows 10 feature release by Microsoft's calculations was the
initial Windows 10 operating system release in June 2015 that was
codenamed "Threshold 1." The November 2015 update, "Threshold 2," was
the second. The Windows 10 Anniversary Update is the third.
At one point, Microsoft was planning to release two Redstone updates in calendar 2016, but earlier this year, officials decided to push Redstone 2 to early 2017, my sources told me back in March.
According to the TechNet article, there will be two Windows 10 feature "upgrades" in 2017, which we've heard are codenamed Redstone 2 and Redstone 3. They'll likely arrive in the spring and fall of 2017, if Microsoft holds to its usual release timetables.