Question:

Did David pray for grace? How many times did he pray for grace?

Answer:

Thank you for writing about David's praying for grace. He did pray for grace, lots and lots of times, although in the Psalms he didn't actually use the word "grace," because there was in Hebrew no exact single word equivalent to the charis of the Greek New Testament. David did use the word for what is perhaps the closest Old Testament equivalent, chen, which means "favor." David prayed many times for God to show His favor and kindness toward him, both for protection of David from his enemies and for God's forgiving of David's sins.

The word "mercy" has a very similar meaning to "grace," and David frequently spoke of (and prayed for) God's mercy. There are several words in Hebrew that are translated "mercy" (usually translated in the Septuagint, an early Greek translation of the Old Testament, by eleos, the Greek word for "mercy"), and you will find the word "mercy" frequently in various English translations of the Book of Psalms.

Here are some examples from the King James Version (each of the following is specifically a Psalm of David):

"But I have trusted in thy mercy; my heart shall rejoice in thy salvation." (Psalm 13:5)

"Great deliverance giveth he to his king; and sheweth mercy to his anointed, to David, and to his seed for evermore." (Psalm 18:50)

"For the king trusteth in the LORD, and through the mercy of the most High he shall not be moved." (Psalm 21:7)

"Surely goodness and mercy shall follow me all the days of my life: and I will dwell in the house of the LORD for ever." (Psalm 23:6)

"Remember not the sins of my youth, nor my transgressions: according to thy mercy remember thou me for thy goodness' sake, O LORD." (Psalm 25:7)

"Hear, O LORD, when I cry with my voice: have mercy also upon me, and answer me." (Psalm 27:7)

"Hear, O LORD, and have mercy upon me: LORD, be thou my helper." (Psalm 30:10)

"I will be glad and rejoice in thy mercy: for thou hast considered my trouble; thou hast known my soul in adversities;" (Psalm 31:7)

"Thy mercy, O LORD, is in the heavens; and thy faithfulness reacheth unto the clouds." (Psalm 36:5)

"But I am like a green olive tree in the house of God: I trust in the mercy of God for ever and ever." (Psalm 52:8)

"Have mercy upon me, O God, according to thy lovingkindness: according unto the multitude of thy tender mercies blot out my transgressions." (Psalm 51:1)

"For thy mercy is great unto the heavens, and thy truth unto the clouds." (Psalm 57:10)

"But as for me, my prayer is unto thee, O LORD, in an acceptable time: O God, in the multitude of thy mercy hear me, in the truth of thy salvation." (Psalm 69:13)

"For great is thy mercy toward me: and thou hast delivered my soul from the lowest hell." (Psalm 86:13)

"For thou, Lord, art good, and ready to forgive; and plenteous in mercy unto all them that call upon thee." (Psalm 86:5)

"But thou, O Lord, art a God full of compassion, and gracious, long suffering, and plenteous in mercy and truth." (Psalm 86:15)

"The LORD is merciful and gracious, slow to anger, and plenteous in mercy.... For as the heaven is high above the earth, so great is his mercy toward them that fear him.... But the mercy of the LORD is from everlasting to everlasting upon them that fear him, and his righteousness unto children's children;" (Psalm 103:8, 11, 17)

"For thy mercy is great above the heavens: and thy truth reacheth unto the clouds." (Psalm 108:4)

"But do thou for me, O GOD the Lord, for thy name's sake: because thy mercy is good, deliver thou me.... Help me, O LORD my God: O save me according to thy mercy:" (Psalm 109:21, 26)

"The LORD is gracious, and full of compassion; slow to anger, and of great mercy." (Psalm 145:8)

One word translated as "mercy" in the King James Version (and New King James Version) is chesed, oft-repeated in Psalm 136:

"O give thanks unto the LORD; for he is good:
  for his mercy endureth for ever.
O give thanks unto the God of gods:
  for his mercy endureth for ever." (Psalm 136:1-2, KJV)

"Oh, give thanks to the LORD, for He is good!
  For His mercy endures forever.
Oh, give thanks to the God of gods!
  For His mercy endures forever." (Psalm 136:1-2, NKJV).

The phrase "For His mercy endures forever" occurs in each verse of the entire Psalm.

Other translations have rendered chesed in this Psalm as "lovingkindness," "love," and "steadfast love," as referring to God's gracious covenant love for His people:

"Give thanks to the LORD, for He is good,
  For His lovingkindness is everlasting.
Give thanks to the God of gods,
  For His lovingkindness is everlasting." (Psalm 136:1-2, NASB)

"Give thanks to the LORD, for he is good.
  His love endures forever.
Give thanks to the God of gods.
  His love endures forever." (Psalm 136:1-2, NIV)

"Give thanks to the LORD, for he is good,
  for his steadfast love endures forever.
Give thanks to the God of gods,
  for his steadfast love endures forever." (Psalm 136:1-2, ESV)

An earlier metrical version of Psalm 136 recognizes that the "mercy" spoken of in Psalm 136 is essentially the same as God's "grace":

"O thank the Lord, for good is He;
  For mercy hath he ever.
Thanks to the God of gods give ye;
  For His grace faileth never." (Psalm 136:1-2)

In that metrical version, "For mercy hath he ever" is regarded as equivalent to "For His grace faileth never." Thus God's mercy in Psalm 136 is taken as equivalent to God's grace.

That David prayed for God's forgiving grace is especially evident in Psalm 51 (where the New American Standard Bible actually renders "Have mercy" as "Be gracious"):

"Have mercy upon me, O God,
  according to thy lovingkindness:
according unto the multitude of thy tender mercies
  blot out my transgressions." (Psalm 51:1, KJV)

"Be gracious to me, O God,
  according to Your lovingkindness;
According to the greatness of Your compassion
   blot out my transgressions." (Psalm 51:1, NASB)

"Have mercy on me, O God,
  according to your unfailing love;
according to your great compassion
  blot out my transgressions." (Psalm 51:1, NIV)

"Have mercy upon me, O God,
  according to thy lovingkindness:
according unto the multitude of thy tender mercies
   blot out my transgressions." (Psalm 51:1, NKJV)

"Have mercy on me, O God,
  according to your steadfast love;
according to your abundant mercy
  blot out my transgressions." (Psalm 51:1, ESV

The men who wrote the chapters in the Book of Psalms were genuine believers; they were already saved. When they prayed for mercy, it was sometimes for the Lord to protect them from their enemies, but also, when they were sorry for their sins, they prayed that the Lord would be gracious and merciful to them and forgive their sins.

Have I answered your questions? I hope so. May the Lord be gracious to you.