You can do this using the sysctl utility:

sysctl -n hw.ncpu

Even easier:

sysctl -n hw.ncpu

This should be cross platform. At least for Linux and Mac OS X.

python -c  import multiprocessing as mp; print(mp.cpu_count()) 

A little bit slow but works.

system_profiler SPHardwareDataType shows I have 1 processor and 4 cores.

[~] system_profiler SPHardwareDataType
Hardware:

    Hardware Overview:

      Model Name: MacBook Pro
      Model Identifier: MacBookPro9,1
      Processor Name: Intel Core i7
      Processor Speed: 2.6 GHz
      Number of Processors: 1
      Total Number of Cores: 4

      <snip>

[~] 

However, sysctl disagrees:

[~] sysctl -n hw.logicalcpu
8
[~] sysctl -n hw.physicalcpu
4
[~] 

But sysctl appears correct, as when I run a program that should take up all CPU slots, I see this program taking close to 800% of CPU time (in top):

PID   COMMAND      %CPU  
4306  top          5.6   
4304  java         745.7 
4296  locationd    0.0  

To do this in C you can use the sysctl(3) family of functions:

int count;
size_t count_len = sizeof(count);
sysctlbyname("hw.logicalcpu", &count, &count_len, NULL, 0);
fprintf(stderr,"you have %i cpu cores", count);

Interesting values to use in place of "hw.logicalcpu", which counts cores, are (from this comment in the kernel source):

• hw.ncpu: The maximum number of processors that could be available this boot.

  Use this value for sizing of static per processor arrays; i.e. processor load statistics.

• hw.activecpu: The number of processors currently available for executing threads.

  Use this number to determine the number threads to create in SMP aware applications.

  This number can change when power management modes are changed.

• hw.physicalcpu: The number of physical processors available in the current power management mode.

• hw.physicalcpu_max: The maximum number of physical processors that could be available this boot.

• hw.logicalcpu: The number of logical processors available in the current power management mode.

• hw.logicalcpu_max: The maximum number of logical processors that could be available this boot.

getconf works both in Mac OS X and Linux, just in case you need it to be compatible with both systems:

$ getconf _NPROCESSORS_ONLN
12

$ system_profiler | grep  Total Number Of Cores 

Use the system_profiler | grep "Cores" command.

I have a:

MacBook Pro Retina, Mid 2012.

Processor: 2.6 GHz Intel Core i7

user$ system_profiler | grep "Cores"
      Total Number of Cores: 4

user$ sysctl -n hw.ncpu
8

According to Wikipedia, (http://en.wikipedia.org/wiki/Intel_Core#Core_i7) there is no Core i7 with 8 physical cores so the Hyperthreading idea must be the case. Ignore sysctl and use the system_profiler value for accuracy. The real question is whether or not you can efficiently run applications with 4 cores (long compile jobs?) without interrupting other processes.

Running a compiler parallelized with 4 cores doesn t appear to dramatically affect regular OS operations. So perhaps treating it as 8 cores is not so bad.

As jkp said in a comment, that doesn t show the actual number of physical cores. to get the number of physical cores you can use the following command:

system_profiler SPHardwareDataType

The following command gives you all information about your CPU

$ sysctl -a | sort | grep cpu

It wasn t specified in the original question (although I saw OP post in comments that this wasn t an option), but many developers on macOS have the Homebrew package manager installed.

For future developers who stumble upon this question, as long as the assumption (or requirement) of Homebrew being installed exists (e.g., in an engineering organization in a company), nproc is one of the common GNU binaries that is included in the coreutils package.

brew install coreutils

If you have scripts that you would prefer to write once (for Linux + macOS) instead of twice, or to avoid having if blocks where you need to detect the OS to know whether or not to call nproc vs sysctl -n hw.logicalcpu, this may be a better option.

Comments for 2 good replies above:

1) re the accepted reply (and comments) by jkp: hw.ncpu is apparently deprecated in favor of hw.logicalcpu (https://ghc.haskell.org/trac/ghc/ticket/8594)

2) re the 2014 update by Karl Ehr: on my computer (with 2.5 ghz intel core i7),sysctl -a | grep machdep.cpu | grep per_package returns different numbers:

machdep.cpu.logical_per_package: 16

machdep.cpu.cores_per_package: 8

The desired values are:

machdep.cpu.core_count: 4

machdep.cpu.thread_count: 8

Which match:

hw.physicalcpu: 4

hw.logicalcpu: 8

CLARIFICATION

When this question was asked the OP did not say that he wanted the number of LOGICAL cores rather than the actual number of cores, so this answer logically (no pun intended) answers with a way to get the actual number of real physical cores, not the number that the OS tries to virtualize through hyperthreading voodoo.

UPDATE TO HANDLE FLAW IN YOSEMITE

Due to a weird bug in OS X Yosemite (and possibly newer versions, such as the upcoming El Capitan), I ve made a small modification. (The old version still worked perfectly well if you just ignore STDERR, which is all the modification does for you.)

Every other answer given here either

1. gives incorrect information
2. gives no information, due to an error in the command implementation
3. runs unbelievably slowly (taking the better part of a minute to complete), or
4. gives too much data, and thus might be useful for interactive use, but is useless if you want to use the data programmatically (for instance, as input to a command like bundle install --jobs 3 where you want the number in place of 3 to be one less than the number of cores you ve got, or at least not more than the number of cores)

The way to get just the number of cores, reliably, correctly, reasonably quickly, and without extra information or even extra characters around the answer, is this:

system_profiler SPHardwareDataType 2> /dev/null | grep  Total Number of Cores  | cut -d: -f2 | tr -d    

On a MacBook Pro running Mavericks, sysctl -a | grep hw.cpu will only return some cryptic details. Much more detailed and accessible information is revealed in the machdep.cpu section, ie:

sysctl -a | grep machdep.cpu

In particular, for processors with HyperThreading (HT), you ll see the total enumerated CPU count (logical_per_package) as double that of the physical core count (cores_per_package).

sysctl -a | grep machdep.cpu  | grep per_package

This can be done in a more portable way:

$ nproc --all
32

Compatible with macOS and Linux.