The plural of the word "sir" is "sirs." The word "sir" is a polite way to address a man whom you do not know well.
Moreover, "sir" can be used in a formal address to a man who holds a higher status than the person using the address. "Sirs" is often used to address a letter when the identity of the person receiving the letter is unknown or if the letter is intended for an organization with multiple board members. The first known use of the word was in the 13th century, and it came from the Middle English dialect.