def _CalculateNumElement(mol, AtomicNumber=6):
    """
    **Internal used only**
    Calculation of specific type of atom number in a molecule
    
    Calculation of element counts with atomic number equal to n in a molecule
    
    :param mol: molecular
    :type mol: rdkit.Chem.rdchem.Mol
    :param AtomicNumber: the AtomicNumber of atom to be counted, defaults to 6
    :type AtomicNumber: int, optional
    :return: the number of stereo centers
    :rtype: int
    
    """
    return len(
            [atom for atom in mol.GetAtoms()\
                if atom.GetAtomicNum() == AtomicNumber]
            )