Is it permissible to seek forgiveness for the deceased? Is there a specific wording for doing so on behalf of others? And is it permissible to seek forgiveness for the living and for a large number of people all at once?

Seeking forgiveness benefits the deceased according to the consensus of scholars. This is supported by the Almighty's saying: "And those who came after them say, 'Our Lord, forgive us and our brethren who preceded us in faith'"; as well as the hadiths concerning the funeral prayer, which include intercession and seeking forgiveness for the deceased. As for seeking forgiveness for the living, it is permissible and recommended. This is indicated by the Prophet's (peace be upon him) supplication, "O Allah, forgive those who shaved their heads," and the hadith, "Whoever supplicates for his brother in his absence, the angel entrusted with him says: Amen, and to you the same." It is recommended to seek forgiveness for any number of Muslims, and it is best to seek forgiveness generally for believing men and women, because "Whoever seeks forgiveness for believing men and believing women, Allah will write for him a good deed for every believing man and woman."