All releases newer than R2013b do not have a limit on the number of workers (CPU cores) a local machine can utilize through Parallel Computing Toolbox. Prior to R2014a, the maximum number of workers was 12.
Please note that the default number of workers Parallel Computing Toolbox will attempt to use is 12. This number can be set higher in the "local" cluster profile. Additionally, if you edit the profile and leave the number of workers as unset, Parallel Computing Toolbox will attempt to use a number of workers equal to the physical cores of the computer.
If you wish to use multiple computers to perform parallel and distributed job processing, MATLAB Distributed Computing Server will be required to utilize computational resources of other machines.
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