St. Mark of Ephesus on the Lord Have Mercies

St. Mark of Ephesus 1392-1444

We say it, I think, twelve times as a symbol of the Apostles, to whom this divine prayer (referring to the “Our Father) was first delivered.

The “Lord have mercy” is said…forty times as customary before the Prayers. The number has a special value, because God is expiated through fasting for so many days. (An Exposition of the Church’s Daily Prayer, 2)