this is what Launchpad [has to say][1] about Ubuntu, so I guess it applies to others: 32 or 64 CPUs for x86 and x86_64 respectively. 


Redhat says the same, but in a [management-friendly table][2]. Redhat EL6 can do 32 for x86, or 128 or 4096 CPUs cores for x86_64.


 [1]: https://answers.launchpad.net/ubuntu/+question/9820
 [2]: http://www.redhat.com/rhel/compare/