Ok so I'm doing MLA for a book and I need help to tell how many authors it has...?

I'm doing a MLA citation for a book by Helen Keller. The book is called 'The Story of my Life'. Helen wrote most of it(the body of the book) herself. but it includes some things added in the front by other people. Like the introduction, it's written by Ralph Barton Perry. Then it has a editor's preface written by John Albert Macy. How may authors are there?? Please help thanks! (: