The word(s) "letter" appears 33 time(s) in 33 verse(s) in Quran in Mohsin Khan translation.

(31) Qaf. [These letters (Qaf, etc.) are one of the miracles of the Qur'an, and none but Allah (Alone) knows their meanings]. By the Glorious Qur'an.

(32) He it is Who sent among the unlettered ones a Messenger (Muhammad صلى الله عليه وسلم) from among themselves, reciting to them His Verses, purifying them (from the filth of disbelief and polytheism), and teaching them the Book (this Qur'an, Islamic laws and Islamic jurisprudence) and Al-Hikmah (As-Sunnah: legal ways, orders, acts of worship of Prophet Muhammad صلى الله عليه وسلم). And verily, they had been before in manifest error;

(33) Nun. [These letters (Nun, etc.) are one of the miracles of the Qur'an, and none but Allah (Alone) knows their meanings]. By the pen and by what they (the angels) write (in the Records of men).