At a minimum, collections of short stories tend to be at least 150 manuscript pages and many are longer. You might include nine short stories, or fifteen, depending upon the length of the individual stories. However, just as putting flour, butter, eggs and sugar into a bowl doesn’t make a cake, a strong collection isn’t created by simply amassing enough pages. You have to get the ingredients to work together. While individual stories can be powerful on their own, they should gain something more by working in concert with other stories.
There are a variety of ways to knit stories together to create a larger whole. In Irene Zabytko’s When Luba Laves Home, each story is about Luba, a young woman growing up in Chicago’s Ukrainian Village. Edwidge Danticat’s Krik? Krak! focuses on life under Haiti’s dictatorship. Raymond Carver’s What We Talk About When We Talk About Love centers on characters avoiding the topic of—what else?—love. There’s really no limit to the ways in which you can create unity. Hannah Tinti’s Animal Crackers includes stories where animals play a significant role. The stories in Lorrie Moore’s Birds of America, are sewn together by the author’s dark wit.
While we’re on the topic, it is tough to publish a short story collection—even more so than a novel. Don’t let that deter you, but make sure you’re submitting your strongest work. And you might try submitting the stories, individually, to magazines. It’s not uncommon for stories to appear in magazines before published as a collection.