The NDK's continuous builds can be accessed by anyone at https://ci.android.com/builds/branches/aosp-master-ndk/grid. aosp-master-ndk is the development branch, and release branches can be selected with the branch name field at the top of the page.
Disclaimer: These builds are not suitable for production use. This is just a continuous build. The amount of testing these builds have been put through is minimal. A successful build only means that our test suite built successfully on Linux and Darwin. Windows is not covered (the Windows build bots are actually Linux), and none of the tests have actually been run yet.
This is the master (development) branch of the NDK. Corresponds to https://android.googlesource.com/platform/manifest/+/master-ndk.
Release branches of the NDK. You can find the build number for the official release by examining the source.properties file in your NDK. The Pkg.Revision entry is in the format MAJOR.MINOR.BUILD (with beta information being appended if appropriate).