How can I check the build number of the current available macOS / OS X installers?
There is an app for that :) https://apps.apple.com/app/mactracker/id430255202?mt=12

It seems like there is no web service or similar providing this info, but I just took a quick look at the app and it seems to provide infos on every mac- and iOS, with build informations for every version.

Screenshot: