Is it permissible to repeat the morning adhkar – such as Sayyid al-Istighfar and saying "Subhan Allah wa bihamdih" a hundred times – to increase good, and is Sayyid al-Istighfar considered one of the types of istighfar that can be repeated multiple times?

There is no harm in repeating the morning and evening remembrances (adhkar) and other invocations and supplications, including the master of supplications for forgiveness (sayyid al-istighfar). Allah Almighty has opened the door of remembrance to His servants and encouraged its abundance. This is indicated by His saying, the Most High: (O you who have believed, remember Allah with much remembrance and exalt Him morning and afternoon). (Al-Ahzab: 41-42) And likewise, the Prophet, peace and blessings be upon him, said: "Let your tongue be moist with the remembrance of Allah," and his saying: "Whoever says in the morning and in the evening: 'Subhanallah wa bihamdihi' (Glorified is Allah and praised is He) one hundred times, no one will come on the Day of Resurrection with anything better than what he came with, except for someone who said the same as he did, or more." Repeating and increasing the adhkar is desirable; rather, one should do it in abundance at all times.