I would like to know, from someone who has marketed a collection of short stories (or is otherwise knowledgeable in these affairs), about how many short stories a publisher is looking for to constitute a collection. Let's assume all the short stories are between two and ten pages long.
Short story collections are notoriously had to place over the transom. Typically a writer will have published most or all of the stories in other venues and then work with an agent to market the collection...at which point the length will vary widely depending on the publisher.
At any rate, there's really no rule of thumb due to the differences between publishers. Do you have publishers in mind? Have you published any of the stories in publications that are likely to be of interest to a publisher?
My rule of thumb: Publish when the paperback is thick enough to put a readable title on the spine.
I use CreateSpace for my paperbacks. For CreateSpace, you need a minimum of 110 pages to put text on the spine. That gives 1/8" of room for the title. That's maybe an 11 or 12 point font (assuming all caps or small caps with no descenders), which is pretty darned small.
So I figure a minimum of 130 pages total, including frontmatter and backmatter.
The amount of text you need depends on a bunch of parameters that affect page count: trim size, font, font size, leading (aka line spacing), margins, paragraph length, amount of frontmatter and backmatter.
I know numerous writers (including me) who publish collections with 5 stories. My one collection (so far) has 27000 words, including 24000 words of story content, an introduction for the collection, and 300–500 of author notes for each story.
I'm in the UK but I've had my first short story collection published at the tail end of 2018 by an American publisher who had previously published academic publications, but was interested in branching out with my collection of fictional stories, which are all cello based, including a lot of historical research. I aimed for between 40 and 45,000 words, and provided illustrations for all the stories, and it was published as I submitted it, already proofread and with a list of illustration attributions and permissions. As I was writing in a field that also is difficult to place I feel very lucky to have found the right publisher for that collection. I have no agent at present, but joined the Society of Authors who give useful advice on contracts.