The numel(M) function returns the number of elements in the array, M, or the number of elements in the subreference M[i,j].

numel does not perform bounds checking of the subreference index to assure that the index is valid.  Therefore, numel(M,5,5) will still return 1, the size of a subreference of the form M[5,5], even for a 2 x 2 array, M.  

The special string, ":" can be used to denote the range of an entire dimension, for example, numel(M,":",1).  Or, ":" by itself as in numel(M,":") can be used to denote the whole array, which is equivalent to numel(M).

$\mathrm{with}\left(\mathrm{MTM}\right)\:$

$M≔\mathrm{Matrix}\left(2\,3\right)$

$M≔\left[\begin{array}{ccc}0& 0& 0\\ 0& 0& 0\end{array}\right]$

$\mathrm{numel}\left(M\right)$

$6$

$\mathrm{numel}\left(M\,\left[1\,2\right]\,2\right)$

$2$

$\mathrm{numel}\left(M\,1\,'':''\right)$

$3$