How May Editors Does it Take?

How many editors does it take to find all the errors in a book?  I feel like I went over my first book with a fine tooth comb and there were still more than six typos one reader found.  The reader's name is Grace and she 'gracefully' (sorry) asked if I wanted to know the typos.  She loved the book and was very kind about the mistakes.  A big help to me.  So much so that she just read my new book (to be released mid June).
This did make me wonder though.  Between me and my co-writer we have about ten people reading the book, looking for mistakes.  None of our readers are professional editors but I would think, between 10 people, all the errors would be found.  But maybe not.  So what is it professional editors do, or have, that gives them the ability to find all the errors?  Or do they?  I'm self published so I use what I have.  What happens in the professional world?  Is there just one editor?  Are there several?  Please, share what you know in the comments.