There is no plural for ma’am. As a form of direct address, it is always singular. Ma’am is the shortcut for madam–when we delete the letter d in madam, we replace it with an apostrophe.
Madam is used to address or refer to a woman–whether married or unmarried–in a polite or respectful way. The word madam comes from Old French ma dame, which means “mylady.” (1)
The plural of madam is madams or mesdames (formal). (2)
Madame, strictly speaking, is a title for a married Frenchwoman. (3) Many, however, no longer make the distinction between madam and madame.
(1) Oxford American Dictionary