this is what Launchpad has to say about Ubuntu, so I guess it applies to others:
1.Intel x86: Maximum CPUs: 32 (including logical CPUs) Maximum memory: 64GB Maximum filesize: 8TB Maximum filesystem size (ext3) 16TB Maximum per-process virtual address space: 4GB 2.AMD64/EM64T: Maximum CPUs: 64 Maximum memory: 128GB Maximum filesize: 8TB Maximum filesystem size (ext3): 16TB Maximum per-process virtual address space: N/A These are standard max limitations whereas Linux cluster systems can scale up to 1024 CPU's. That is 32 or 64 CPUs for x86 and x86_64 respectively.
Redhat says the same, but in a management-friendly table. Redhat EL6 can do 32 for x86, or 128 or 4096 CPUs cores for x86_64.
