For most nouns in the English language, adding an "s" to the end of the singular version of a noun creates a plural version. In this way, one dog becomes two shirt becomes three shirts...etc. It is a very basic concept of any language to indicate specifically when you have more than one of something by changing the noun's form.

Some commonly used clothing nouns, however, have no singular/plural forms. They are used in an uncountable way naturally and as such appear in only a form which appears plural. For example:
  • clothes
  • pants
  • glasses
  • shorts

The above nouns can be made plural by adding a counter, i.e. 2 pairs of pants, 4 boxes of pants, etc. However, one cannot say that they have five clothes, two pants, one pant, etc. These would all be incorrect grammatically and would likely not be understood by the person to whom you are speaking.

It is, however, very common to hear non-native speakers talking about their "clotheses", or "pantses". Be aware that this is wrong, and remember that if you do want to make them plural, you must add a plural counter, such as "pairs", "items", "units", "boxes", etc.