Once, and this includes my saying 2 Kaddishim on the Ommud + 2 mourner’s Kaddishim, each only recited once, getting “L’Eilo L’Eilo MiKol” correct 1st time around each time.
But I shan’t have to worry about saying L’Eilo twice by mistake after Yom Kippur, as I am terminating my 11 months of saying Kaddish in the middle of the Asseres y’Mei Teshuoh.