What is plural noun form of pajamas?

The noun pyjamas (pajamas) is both the singular and the plural form.

The noun pyjamas is an uncountablenoun of a group called binarynouns, Words for something made up of two parts that function as one. Binary nouns are a short form for 'pair of'. Example: one pair of pyjamas, two pairs of pajamas.

Other binary nouns include pants, shorts, glasses, binoculars, scissors, tweezers, etc.