I am saying Kaddish for someone who passed away on ח׳ בְּנִיסָן תשפ״ג, and was buried the next day. On what date and which Tfilla do I stop saying Kaddish?

Minchah of ח' אדר ראשון is the last Tefillah you say Kaddish. In your case the count is from the day of burial, based on מאמר קדישין in באור הלכה on שולחן ערוך סימן קלב. This is according to the Ashkenzi Minhag that you stop saying Kaddish after 11 months.