Even though pants are one single item of clothing, they have two leg parts. Because of this, we always say "pants" with an "-s" at the end, and if you want to count them, you call them "a pair":

Why do you have so many pairs of pants?

You also call these clothing items "a pair":

a pair of glasses

a pair of gloves

a pair of shorts

a pair of shoes

a pair of socks