Do I need to reintroduce the characters in each story for a short story collection?

If It's all bound in one package I would think not. Short story collections I've read by Andrzej Sapkowski and Joe Abercrombie didn't. Maybe if the stories might appear elsewhere independently, but even then you would want to avoid redundancy.

Depending how they are written, the stories might not need to reintroduce characters anyway, they might give different information about the characters. After all, we never get every bit of information about a character in a story of any given length.

I also think that one introduction is enough. If the stories have been published elsewhere, there's probably a bit of an introduction in each one. If they're all new stories, I imagine that you wouldn't need to introduce the same characters each time, any more than you'd start off each chapter in a book with a recap of who's who.