Rise, oh penitent worshiper, and receive the fruits of your groveling, for yours is an excellent question.
First of all: You’re doing it right. Writing requires practice, and if you feel that writing and publishing short stories is the way to building up the skills you need and honing your craft, then good on you.
In a query letter, I do actually look at how many stories/books/articles a writer has previously published, as well as where they have been published. But it’s a careful balancing act. If you’ve published nine stories, but only three of them were in what you consider to be prestigious publications, then focus on those three. For example:
“I have published nine short stories through various magazines and publications, such as The Atlantic, Ploughshares, and McSweeney’s. A complete listing of my publishing history can be found in my resume, attached.”
I used The Atlantic, etc. because they are journals I assume the whole class will recognize, but if you are writing for a specific genre, you can trust that the editor/agent you’re querying will recognize the big journals in that genre (The Magazine of Fantasy & Science Fiction if you write SF&F, for example).
Also, don’t be intimidated by the fact that I mentioned hugely prestigious journals and a robust publishing record. If you have only published one short story in your college’s literary journal, it’s worth mentioning. But keep in mind that if you’ve only published one short story in your college journal, then two would look better, and three would look even better than that. The point being: If you’re worried that your publishing record is short and unimpressive, then maybe you should get a little more practice under your belt (by publishing more short stories/articles) before querying a publisher about a book.
What the above example does is tell the editor/agent what they need to know in as succinct a manner as possible. From there you can elaborate, either by continuing the paragraph or by including the necessary information in your resume. If you have a ton of prestigious publications under your belt, be aware of space constraints and don’t try to pack it all into your query letter.
Class dismissed. You’re all going to be awesome. :)