In MATLAB versions up to 7.7 (2008b) you can determine the maximum number of threads that it can use with

N = maxNumCompThreads
LASTN = maxNumCompThreads(N)
LASTN = maxNumCompThreads('automatic') 

In MATLAB versions 7.8 and onward, the maxNumCompThreads function is not supported any more. Instead, you can tell MATLAB that it should use a single thread at startup by providing the -singleCompThread to the smartup command, e.g.

matlab78 -nodesktop -singleCompThread 

See for more details.