How many huffaz died at the time of Umar which led him to preserve the Qur'an?

In a Nutshell: There are different narrations that answer this question all without a precise answer. The consensus suggests that there were at least 70 hafidhs (memorisers) of Qur'an who died at Yamama and similar battles leading to Umar (ra) instructing the Qur'an be compiled in order to be preserved. The strongest evidences on this matter appear below. قتل منهم يوم أحد سبعون ، ويوم بئر معونة سبعون ، ويوم اليمامة سبعون It was narrated Qatada narrated from Anas (ra), "Seventy from the Ansar were martyred on the day of Uhud, and seventy on the day (of the battle of) Bir Ma'una, and seventy on the day of Al-Yamama." (Sahih al-Bukhari 4078) The event of gathering all the Qur'an's surahs in one book was after the battle of Yamama at the time of Abu Bakr. As Abu Bakr (ra) said: إِنَّ عُمَرَ أَتَانِي فَقَالَ إِنَّ الْقَتْلَ قَدِ اسْتَحَرَّ يَوْمَ الْيَمَامَةِ بِالنَّاسِ، وَإِنِّي أَخْشَى أَنْ يَسْتَحِرَّ الْقَتْلُ بِالْقُرَّاءِ فِي الْمَوَاطِنِ فَيَذْهَبَ كَثِيرٌ مِنَ الْقُرْآنِ، إِلاَّ أَنْ تَجْمَعُوهُ "Umar has come to me and said, The people have suffered heavy casualties on the day of (the battle of) Yamama, and I am afraid that there will be more casualties among the Qurra' (those who know the Qur'an by heart) at other battlefields, whereby a large part of the Qur'an may be lost, unless you collect it." (Bukhari 4679)