بسم الله الرحمن الرحيم
(Fatwa: 176/131/B=2/1439)
This number is not proved from the Quran and Hadith. It is the experience of elders. Someone tells to read it seventy thousand times and someone tells to read it one lakh twenty five thousand times. The most excellent dhikr is the Holy Quran, it should be recited and then isal-e-sawab and dua for forgiveness should be made for the dead. At least recite Surah Ikhlas 11 times and do isal-o-sawab.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband