Over 11,000 people subscribe to the WartHogs** mailing list (the book is set at WartHogs). The (church I belong to also in the book) boasts 4 million members. I teach at (name of college) and have wide connections to the (church) world, etc, etc. [That is, should I try to suggest the potential audience a la a typical nonfiction book?]
You can certainly suggest the size of the audience, but the only thing publishers are really interested in is if you have something in place NOW to reach them, and in fact you've been reaching them before this book gets published.
Thus: "this book will appeal to all women" is hilarious because the unspoken reply is "do you have email addresses and personal connection to all women?"
One of the most heartbreaking things I have to do disabuse writers of how many people they "know" will actually shell out money to buy their book.
It's one of the most shocking moments of a writer's career when they realize that not all 10,000 members of a mailing list will be interested or hooked enough to buy a book that speaks directly to the interests of that list.
(this is one of the places where "they're not buying this because my books sux" can REALLY creep in)
Being on a mailing list is not the same as having access to the mailing list; having access to the mailing list is good but it's only the start. Studies show (and your own buying habits if you track them will verify) that people buy books based on word of mouth. In other words, those people on the mailing list have to be talking about this book for it to be an effective part of your platform.
So, you might ask, why will people be talking about your book? They'll talk about it if they know you. "Oh look Prunella Smithee has a book coming out. And it's all about WartHogs School of Deportment for Young Ladies!"
What does that mean for you? It means the 10,000 person mailing list is more effective if you are a regular contributor. It's MOST effective if it's YOUR mailing list: ie people who have signed up to hear from you.
Building a mailing list takes enormous time and effort. Maintaining it takes even more time. There are some nifty electronic tools out there to help you. My guess is some contributors to the comment column of this blogpost will offer up some ideas for you.
Blog readership is also a good piece of your platform. One of the things I always look at when I'm checking out blogs is how many people comment. More = better (unless they're all idiot trolls.)
Another tool is GET KNOWN BEFORE THE BOOK DEAL by Christina Katz. I don't rep this author and didn't sell this book so I've got no vested interest in whether you buy it, but you should.
You don't need platform for novels but it doesn't hurt if people know you before hand.
You DO need platform for non-fiction, and that includes memoir.
The number of people on a mailing list, or who belong to a church, does not translate to how many people will buy your book. I wish it did.
**obviously not the name of the actual place, but you knew that right?