Přesně takhle
$this->db->select_max('el_order');
$res = $this->db->get('elem_diccio')->row()->el_order;
$eldi_key = url_title($this->input->post('id'), 'underscore', TRUE);
$el_order = $res+1;
$datos = array(
'ID' => $id,
'el_order' => $el_order,
'name' => $this->input->post('name'),
);
$this->db->insert('elem_diccio', $datos);