Is it permissible to recite Surat Al-Ikhlas one hundred times in a single rak'ah of a supererogatory prayer?

There is no harm in repeating a surah (definition) multiple times in a single rak'ah or across two rak'ahs, especially Surat "Qul Huwa Allahu Ahad" (Say: He is Allah, the One) due to its great virtue. The Prophet (peace be upon him) said: "By Him in Whose Hand is my soul, it is equivalent to one-third of the Qur'an."

However, two matters should be observed: 1. The repetition should not be a specific number; rather, it is legislated without designation, because specifying a number is an innovation (bid'ah). 2. One should not habitually repeat a specific surah every night, as this is not legislated. Rather, one should do so occasionally and not constantly. It is preferable to recite a different surah in each rak'ah, though there is no harm in repetition, based on the Almighty's saying: (So recite what is easy [for you] of the Qur'an).