When an author is deceased and his or her series is continued by a different author, does the numbering of the series continue?

The book in question is ROBERT B. PARKER'S IRONHORSE written by Robert Knott. Parker wrote the first 4 books in this series ( Virgil Cole & Everett Hitch). So would this book be the fifth, even though not written by Parker?

I would think so. There are series that are alternated between more than one living author so I don't see why the numbers couldn't conitnue when one dies and it's taken over by another author. 

I would see how goodreads, fictfact.com and fictionwise.com count the book.

Mary's answer is good -- we generally look at how other trusted websites have set up the series.  There's no blanket rule.

If you enjoy the editing process, you could contact the Admin Team and ask to be placed on the team of book data edit volunteers.  There's lots of editing info/suggestions there, and a place to discuss editing detalis.