def num_taxonomies_by_site(self):
        """
        :returns: an array with the number of assets per each site
        """
        dic = general.group_array(self.array, 'site_id')
        num_taxonomies = numpy.zeros(self.tot_sites, U32)
        for sid, arr in dic.items():
            num_taxonomies[sid] = len(numpy.unique(arr['taxonomy']))
        return num_taxonomies
:returns: an array with the number of assets per each site