Assalamu'alaikum.I want to know what is the plural form of 'Alaihissalam? Thank you for reading and answering this.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The plural form of the statement “Alayhi As-Salaam” (i.e. peace be upon him) is “ʿAlayhim As-Salaam” (i.e. peace be upon them), with the Meem letter indicating the plural form. In a Hadith whose chain of transmission is “good” by some scholars, the Messenger of Allah said, “Confer peace and blessings upon the Prophets and Messengers of Allah, for Allah has sent them as He sent me.” So, whoever mentions a group of the Prophets and wants to confer peace and blessings upon them, he should say, “ʿAlayhim As-Salaatu Wa As-Salaam”, with the Meem letter.

Allah knows best.