HAVE MERCY upon me, O God, according to Your steadfast love; according to the multitude of Your tender mercy and loving-kindness blot out my transgressions. (Psalm 51:1 AMP)
David is asking for forgiveness for adultery with Bathsheba and the murder of her husband. He had a multitude of sins that needed a multitude of tender mercy. Mercy blots out the bottomless pit of our ugly sin.
Also there is a multitude of people needing a multitude of mercy. God has tender mercy for each of us in the multitude. He showed it on the first Passover.
There’s a verse in Psalm 51 that says “Scrub me with hyssop (a plant with small white flowers)
and I shall be clean. Wash me and I shall be whiter than snow.” Hyssop was used at the first Passover to apply the blood of the lamb to the door. There was a multitude of mercy for the multitude of the children of Israel. David needed some of both kinds of mercy.