1.) Author (1-2) - So the author writes the book. Fair enough. They're doing the bulk of the heavy lifting of bringing an idea to life. Sometimes two co-authors work in concert to produce a book. It's exceedingly rare to see more than two collaborators outside of an anthology.
2.) Author's Support Staff (Business) (0-3?) - At a certain level of wealth and/or fame an author will be able to afford to hire employees. I'm not especially familiar with this part of the business, but I do know a few authors who have personal assistants. I'd be surprised if even a Stephen King-level author needs more than a person or two to answer the phone, manage their schedule, etc., but as I said, I'm not particularly familiar with this part of the business.
3.) Author's Support (Personal) (1-10?) - That being said, authors require support and will get it from friends and family. To be frank, in many cases the only reason you are able to read a book is because an author's spouse or other loved one is the breadwinner of the family and supports them financially. But even if the author also works a day job, they will still require emotional support.
4.) Critique Group (0-10) - Some authors find value in a formal critique group. A critique group is basically as large as you want it to be, and you can exchange chapters or (occasionally) whole manuscripts for review.
5.) Beta Readers (1-4) - With few exceptions, most authors use beta readers (also occasionally called alpha readers) which is a trusted reader, possibly another author, who will point out major issues in a first draft of a manuscript.
6.) Literary Agent (0-1) - Many authors end up choosing the traditional publishing route. In this case, they will have a single agent who represents their interests and sells their books to publishers.
7.) Agency Staff (0-5) - Even though an author generally only has one agent, agents don't do all the work of their agency. Sub-agents sell foreign, audio, film, and television rights. Assistants and interns sift through the slush. If you got an agent, it's probably because of the efforts of one of these unsung heroes.
8.) Editor (1-3) - The role of the editor is to correct a book's issues before it is published. There may also have been an acquiring editor whose role was essentially to agree to take on the book with the publishing house. A book can go through multiple rounds of editing, variously referred to as content, line, proofreading, and a variety of other terms. Sometimes as many as 5 people will review your book before it's published.
9.) Cover Artist (1) - I didn't really get into this with #6 above, but you're really a buffoon if you think you can edit your own work or make your own covers. The cover artist is the person either the author pays if self-publishing or the publisher pays (or possibly keeps on staff.)
10.) Marketing Staff (0-12?) - Self-publishers can hire marketing professionals in the form of publicists, blog tour coordinators, and the like. Large publishers can have whole departments dedicated to this (although whether your book will get any love from marketing is another matter.)
11.) Publisher's Business Staff (0-hundreds) - Then, of course, there's the rest of the publisher's staff. Secretaries, accountants, web developers, interns, lawyers, operation staff, and all the other people who make any business hum. A small publisher might only have a dozen employees, or in the odd case, just one. A major publisher will have hundreds, maybe thousands, though not all working on your book, of course.
12.) Bookbinders and Distributors (?) - In the post-Amazon world it's easy to think of books as just showing up at your door. But, believe it or not, even if you're self-published and go entirely through CreateSpace, there's a staff who physically produces your books and ships them out the door to whoever orders them. In the more traditional world, there are staffs in binderies who distribute books to booksellers and then pulp the remainders. Actually, the pulping might be a whole separate process, I'm not even sure. This whole aspect of the industry is something I'm woefully uneducated on. If you're involved in the actual book production process, please chime in in the comments!
13.) Booksellers (1-thousands) - Again, this could be as "simple" as Amazon - but bearing in mind that Amazon has hundreds of thousands of employees - or as complex as a whole legion of big-name bookstores like Barnes and Noble, Books-A-Million, and so forth, as well as small independent bookstores and even places like Wal-Mart or airport news vendors where books are sold.
14.) Librarians and Library Staff (0-thousands) - Typically after the regular booksellers, but sometimes at the same time, libraries will acquire books, either by purchase or donation. Of course, if you make no effort to end up in libraries you never will.
15.) Reviewers (0-∞, probably around 50-100) - Reviewers, both professional and amateur, are critical to a book's success. Publishers may pay for reviews through magazines and services like Kirkus. Alternatively, publishing staff or authors themselves can reach out to reviewers everywhere along the scale from "mass market magazine or newspaper" to "guy with an Amazon account." Many amateur reviewers have followings on their blogs to rival the mass media of yesteryear, so this whole process is kind of a crap shoot.
16.) Other Authors (0-dozens) - Once you've become part of the publishing world, it's easy for an author to reach out to peers, and relatively easy to reach out to successful authors. Other authors will become some of your best champions, both in honing your skill and promoting your work, as well as the ordinary business of friendship. Authors who don't become part of the community flail. Authors who do will often flourish. It's not strictly accurate to say writing is an apprenticeship system, but there is a major element of seasoned authors helping newbies, who in turn help future newbies because they were helped once upon a time.
17.) Professional Organizations and Awards Juries (0-hundreds) - Professional guilds, unions, and associations exist for virtually every genre and type of writer. Such groups range from little more than fraternal backslapping clubs to indispensable sources of insurance, professional contacts, legal counsel, and other services. There's nothing quite comparable in the book world, but union membership is a de facto requirement for professional screenwriters. Organizations like the HWA, RWA, and SFWA also often sponsor industry awards, and while largely made up of authors and volunteers, some of these groups have paid staff as well.
18.) Readers (0-∞, completely impossible to predict) - The final and most important part of the publishing ecosystem is the reader. It could be just mom. It could be an audience of millions. There's really no way to tell, or, honestly, to predict. (If there were, every book would be a bestseller.) Without readers shelling out a few bucks for a book, nobody else gets paid. Not the booksellers, not the publishers, not the agents, and certainly not the author. So yay for you, because if you're reading this, you're the most important person in the world to me: a fan or potential fan.
So, there you have it. What often seems like the work of an individual will actually go through anywhere from half a dozen to hundreds of hands before it's complete.
What do you think? Anything I missed? Anything I completely bungled? Any more thoughts on one of the groups I described?