where do you see these "git numbers" ? git is not svn...
It is from github at the very top of the page where it says "#### commits ## branches ## releases ..." an example would be from your fork "git 566" is "commit fade2b74". And yes it is very confusing. 
COMMIT NUMBERS--
I usually refer to commits/builds by the commit number. It is at the upper left of the page. Commit numbers are sequential. The do not, however, help with git "checkout" commands, like the hash value on the upper right. I still do not understand git checkout syntax well, but I use the commit number when referring to a specific build between releases while posting.
--scryptr
To clarify, that is the commit count, not an index, and is a GitHub feature, nothing to do with git. This number is mostly useless as it can be invalidated in a number of ways. With git, generally we refer to a commit by the first 7 chars of the sha1 hash.
Example:
r69 -> 989737b6a5149f1539fa6783106dd755ebeaeb7c -> 989737b