Sayyiduna Irbad bin Sariyah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) would supplicate for forgiveness three times for those in the first row and once for those in the second row.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب قيام المأمومين خلف الإمام وما فيه من السنن / 1558