We used to count that the Messenger of Allah ﷺ would say in a single sitting, "My Lord, forgive me and accept my repentance, for You are the Ever-Accepting of repentance, the Most Merciful," one hundred times.
حدثنا علي بن محمد حدثنا أبو أسامة والمحاربي عن مالك بن مغول عن محمد بن سوقة عن نافع
إن كنا لنعد لرسول الله ﷺ في المجلس يقول رب اغفر لي وتب علي إنك أنت التواب الرحيم مائة مرة
[Sahih] Reported by Abu Dawud, Prayer, chapter on seeking forgiveness, no. 1516, from the hadith of Abu Usama, with this same wording. Tirmidhi said, no. 3434, hasan sahih gharib. Sufyan bin Uyayna also narrated it from Muhammad bin Suqa with this same wording, and Ibn Hibban graded it sahih, no. 2459.