I need a car lock. (1 lock per car)
We say Caps Lock or Capitals Lock as a short form of Captial Letter- we often use it as a plural, so we keep it in the plural- write the heading in capital letter/in capitals. It is a noun and when placed before Lock, it is a noun functioning as a kind of adjective. These are normally used in the singular, but can be plural if the plural form is the form most commonly used.