http://hg.mozilla.org/try appears to have over 3M commits, and probably in excess of 100k heads (effectively git branches, although I don't think git has any proper term for a commit with no children that isn't referred to by a branch).

Strictly speaking, it's not actually the main project repository (which has closer to 600k commits), but the repository that contains what is effectively all of the pull requests for the past several years (more specifically, all the changes you want to test in automation).

The closed-source monorepos of Google (perforce IIRC), Facebook (Mercurial), and Microsoft (Git) are all going to be far larger than any open-source repository, of which Linux is in the largest size class but not the largest (I believe Chromium's the largest open-source repo I've found).