As many as are needed to make the point, and no more.

Never having completed a book before I haven't a clue. But, in my book, I plan to use larger than average fonts for people like me whose eyesight is decreasing. Maybe you could look at some existing books and see if they give a word count or do a page count and take the average? Bear in mind you can make more than one point.

I've been doing 60,000 to 100,000 for different books. I was just wondering if any of you had some king of magic "golden mean." (A magic number of words that makes publishers eyes light up, I guess.)

Depends on the direction of the publisher. Get an agreement ahead of times on the number of words and photos. Gardening books come in different formats and uses. Many gardening books are 50 to 60 thousand words (includes index, table of contents).

Too many words is a pain to editors. Everyone is always over the number of words for articles, columns, books,etc. and then complain when editors 'slash' out info. If you don't want a hatchet job on your writing then give the editor a clean copy coming in at the expected word count. The less tinkering by the editor the happier he is and the happier you will be.