How many maximum CPUs does Ubuntu support by default?

kernel

_{I think this is kernel dependent and probably will change over time depending on the kernel a release uses, correct me if wrong}

I'd like to know two things for all the currently supported Ubuntu releases:

How many maximum CPUs can Ubuntu handle (by default) at the same time with a standard desktop kernel? Is that count different from a server kernel and 32bit/64bit systems?

Where can I find that information on my system?

Maximum CPUs (i.e. cores) supported by default:

Note: by 32-bit/64-bit we mean the common i386/x86_x64 architectures; PPC, ARM, etc. are not included.

12.04 LTS (and later):

Desktop/Server 32-bit: 8 cores/CPUs
Desktop/Server 64-bit: 256 cores/CPUs (but LiveCD supports 64 by default)

11.10 and below, including 10.04 LTS

Desktop/Server 32-bit: 8 cores/CPUs
Desktop 64-bit: 64 cores/CPUs
Server 64-bit: 256 cores/CPUs

How to find the maximum supported by your running kernel:

As @otus indicated, open a terminal with Ctrl-Alt-T, and type:

grep NR_CPUS /boot/config-`uname -r`

The below is a somewhat technical discussion on what maximum really means:

The "default" maximum is not the maximum!

You can recompile the desktop or server kernels to support up to 512 CPUs for 64-bit (8 is max for 32-bit).
but even those are not absolute maximums!:
- The kernel has "experimental" options for going higher than the "default" maximum
- The limit for 32-bit with BIGSMP=y is 512
- The limit for 64-bit with MAXSMP=y is 4096 (or more!)
- Just because the kernel supports so many cores on the general i386/x86_64 architecture does not mean your hardware will!

All I know for sure is that precise (12.04) desktop 64-bit -generic kernel supports maximum 256 cpus (by default).

You can find the number of CPUs your kernel config supports by doing:

grep CONFIG_NR_CPUS /boot/config-`uname -r`

It all comes down to how one would define physical CPUs. Are they equivalent to sockets, so a multicore and/or hyperthreading CPU is counted as a single socket?

The Ubuntu kernel is configured to support 8 processors / cores in 32-bit and 64 processors / cores in 64-bit.¹

As far as finding that information, I have used Sysinfo and I'm very happy with it. If you don't have it installed, you can do so by just pressing Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.

sudo apt-get install sysinfo

Also to find the Number of processors in a system using terminal, just type:

dmesg |grep processor

I will keep researching this until I find the perfect answer.

^{¹Source:Wiki Answers}