Which one is the plural form f ‘Commandar-in-chief’?
If compound noun contains several words, “s” comes to join with the main part of that noun.

He became rich overnight. What part of speech is 'overnight'?

The nightingale is a singing bird. In this singing is a/an --

I Came home after the rain -----.

The Doctor took the round in the hospital. Here 'round' is a/an ---

Alumni is the plural from of