Codeigniter, mysql, select_max и добавете 1, преди да вмъкнете друг запис

Точно така

 $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);