8 spaces tends to be the de facto standard tab width. But because tab is it's own ASCII character (0x09), much like how space (0x20) and the lower case letter 'a' (0x61) are, it means it's up to whatever text processors used (eg diff, vim, GitHub web application, etc) to interpret a tab character by whatever width they choose. Which is why tab spaces are a per-application setting, eg:

    diff --tabsize=4 ...
    vim "+set tabstop=4" ...
    http://github.com/.../file?ts=4