While creativity on the author’s part typically determines what they want their work to become, ultimately there are outside forces that make the final call.
Often the publisher will have the final say in all things concerning the length and print run of a book, but don’t think it’s a snap decision. Nope. It’s based on a “catch 22” formula. A previously published writer is much more likely to be subsequently published. Also, if their first book sells well the author will almost always be signed to a second and third book deal, but that doesn’t determine if it will be a stand-alone book or part of a series. Still, a publisher can yank the contract and cancel the third book even if the second book’s sales are doing reasonably well. Go figure!
So, what is the measuring stick? The strength of the characters. Limp noodles caught in the hurricane that is your story won’t make it in the publishing industry. Period. You have to breathe life into your characters, give them desires, strengths, faults, and dreams. Your characters have to be bigger than life, ready to step off the pages and get in the reader’s face if necessary. Characters that evoke the entire range of emotions in the reader are more likely to be around for another book than characters that are as flat as the paper they’re written on.
What if you don’t want to write a running series? Well, writing crappy characters just means that you don’t get published, so what you have to do is write a tight story with good characters and have it wrap up in one nice neat little package so that it doesn’t lend itself easily to a sequel.
Still odd things can happen, and all or none of this may apply when you go to publish your first or next book.