Since you're talking about the max. physical (not logical) processors supported, I can say that the numbers for Windows 8 Core (1) and Pro (2) are most likely correct. I'm sure this sort of limitation is for the client versions of Microsoft's OS only, and not the server versions.
As for official documents, other than the About.com page that mwong mentioned (with no sources listed, which Thurrott also omits), I have so far not found any technical documents regarding the limitation (all I could find were older ones related to Windows 7). What I did find however were legal documents that specify the number of physical processors the two versions are licensed to run on, so most likely the code enforces the legal limit as well.
What are these legal documents? They are the EULAs we all ignore while installing Windows. If you don't want to bother running Setup to confirm, you can download PDF copies here.
Windows 8 (Core) Software License Agreement
Windows 8 Pro Software License Agreement
The EULA for Windows RT does not seem to explicitly mention the number of physical processors supported. Windows Server (2012) versions are licensed depending on the number of processors installed (each license = 2 physical processors, so no. of licenses required = ceil(no. of physical processors installed / 2)).