With the forthcoming service packs for SCCM 2012 and SCCM 2012 R2, you can now scale a hierarchy to 600,000 managed devices (50% increase from 400k) and a standalone primary site now can manage a maximum of 150,000 devices (also 50% increase from 100k).
We’ll be testing these scalability numbers over at ITS Partners in the coming weeks and I’ll update this post with our findings.