Is it permissible to make up for istighfar (seeking forgiveness) for past days, whether with the same daily designated number, or more, or less, and what is the ruling on making up for istighfar from old days?

It is permissible to make up for missed remembrances (adhkar), whether few or many. There is no harm in seeking forgiveness (istighfar) any number of times, as it is a righteous deed and a prescribed form of remembrance. Allah commanded it through the tongue of Noah (peace be upon him), and the Prophet (peace and blessings of Allah be upon him) encouraged its abundance.