Supplicating for the Prophet (peace and blessings be upon him) in abundance is among the best deeds which bring us closer to God and one of the greatest acts of piety. It is sanctioned by the Quran, the Sunnah, and the consensus of the Islamic community.

The Quran

God says, "Indeed, God confers His blessing upon the Prophet, and His angels [ask Him to do so]. O you who have believed, ask [God to confer] blessing upon him and ask [God to grant him] peace" (Quran 33: 56).

The Sunnah

• Ubay Ibn Ka'b (may God be pleased with him) said, "In the second third of the night the Prophet (peace and blessings upon him) would say, 'O people! Make remembrance of God, make remembrance of God. On the Day when the blast [of the Horn] reverberates and it is followed by a second blast, death has come with all that it entails, death has come with all it entails.' I said, "O Messenger of God! I supplicate for you in abundance. So how much of my supplications should I make for you?" The Prophet (peace and blessings be upon him) answered, 'However much you wish.' I said, "A quarter?" He replied, 'As you wish; if you do more it is better for you.' I said, "Two thirds?" He said, 'As you wish but if you do more that it is better for you.' I said, "Should I make all my supplications for you?" He said, 'You will be relieved of your worries and your sins will be pardoned" (Al-Tirmidhi and al-Hakim).

• Aws Ibn Aws (may God be pleased with him) narrated that the Prophet (peace and blessings be upon him) said, "One of your best days is Friday; on this day Adam was created and on this day he died; on this day the horn [signaling the end of the world] will sound; and on this day everyone will die. Therefore make supplications for me in abundance for your prayers will be presented to me." He was told, 'O Messenger of God! How are our prayers presented to you after your body has dispersed into the earth i.e. you are dead. He said, ‘God Almighty has prohibited the earth from consuming the bodies of prophets, peace be upon them'" (Ahmed and Abu Dawud, Ibn Khuzaymah, Ibn Habban. Al-Hakim declared it authentic).

Opinion of scholars

Scholars have maintained that the minimum amount of making supplications for the Prophet (peace and blessings be upon him) in abundance is a thousand times; others have maintained that it is three hundred. The luminary, al-Mutaqqi al-Hindi, authored a book on the subject called “Hidayat Rabbi 'ind Faqd al-Murabbi” in which he discussed what a person is to do when he lacks someone to educate him [in religious affairs] and guide him towards the path of God. According to him, at such times a person is to supplicate for the Prophet in abundance — a thousand times at least each day.
Anas Ibn Malik narrated that Prophet (peace and blessings be upon him]) said, "Whoever supplicates for me a thousand times each day will surely reserve his place in paradise before he dies" (compiled by ibn Shahin in “Targheeb al-Diya` fi al-Ahadeeth al-Mukhtara”). Though its chain of transmission is weak, the hadith is put into practice because it is related to good deeds and not legal rulings.
In conclusion, all efforts at making supplications for the Prophet (peace and blessings be upon him) in abundance are meager considering his right upon us and his exalted status before his Lord. The Prophet (peace and blessings be upon him) said, "Whoever supplications for me, the angels will invoke supplications upon him; so you may either increase or decrease your supplications" (Ibn Majah and Ahmed. Al-Mundhir and Ibn Hajar declared it fair).
Imam 'Ali Zayn al-'Abdin, al-Hussein's son (may God be pleased with them both) affirmed that making supplications for the Prophet (peace and blessings be upon him) in abundance will remain a distinguishing feature of orthodox Muslims. He said, "The sign of orthodox Muslims is making supplications for the Prophet in abundance" (Abu al-Qassim al-Taymi in “Al-Targheeb wa al-Tarheeb).
And God Almighty knows best