SELECT k.nazwa_kl FROM klienci2 k join zamowienia2 z on k.id_kl = z.id_kl having count(z.id_kl) >
  ( SELECT  count(z.id_kl) FROM klienci2 k join zamowienia2 z on k.id_kl = z.id_kl WHERE k.nazwa_kl = 'Zaklady C' )
  group by k.nazwa_kl;