My rejection/sale ratio depends on how you look at it, and does tie in with number of rejections for a given story. Overall, I sell roughly ninety percent of the short stories I write, and always have. I'm not sure of the percentage, but the majority sell to the first market I submit them to, largely, I think, because I have a market firmly in mind when writing a story. A market I've studied thoroughly, and where I've read a LOT of issues.
But some of my biggest sales have come after a story was rejected from fourteen to twenty times. In each of these cases, however, I did have to change one or two small things to make the story a closer match to the market.
A prime example of this is a story I wrote called "The Parachute". It was a humorous tale of a teen protagonist basically getting himself into humorous trouble in small town Indiana, and contained several autobiographical incidents. It wouldn't sell anywhere, and gathered, oh, fourteen or fifteen rejections. Even the very low pay and no pay markets rejected it. Nearly every rejection was the same. "Great story, but just isn't right for us."
Then I chanced on guidelines for Sports Afield, an adult hunting/fishing magazine. I read some of their stories, loved them, but knew I needed a new element to make mine fit. I changed the first paragraph to make the teen want a rifle or a shotgun more than anything, but having parents who were afraid buying him one would just get him into trouble. I then told the story exactly as it was told before, but changed the ending to one where, after all the humorous trouble the boy gets into, the parents decide buying him said rifle or shotgun might keep him out of trouble,
Sports Afield bought the story, and paid me an even one thousand dollars for it, which was a LOT of money twenty years ago. That story taught me a serious lesson in why magazines buy particular short stories, and that fit is crucial.
I'm a firm believer in Heinlein's Rules, so I pretty much never shelve a short story. As long as I can find any potential market, I keep the story in circulation. It would be a pity to shelve a story when the next editor down the line might love it. I know one writer who made a major short story sale after the story gathered more than fifty rejections.
There have been very rare exceptions when I eventually gave up on a story after the number of rejections hit the ceiling. I threw away every trace of these stories. But almost all have bubbled back up in my mind, I rewrite them from scratch, and they're usually better the second time around.
You simply can't sell a story you shelve, and many rejections does not automatically mean a story isn't publishable. It often means no more than you haven't yet found a magazine where the fit is close enough to warrant a sale. Or a market where you catch the editor in the right mood. Or a market where the editor has an unusually large percentage of crap in his slush pile for three months running, and while your story isn't going to win any awards, it's better than all the other crap he's been reading.
My general attitude is that I have short stories I've sold, and short stories I'm going to sell somewhere, sometime, somehow, if only I keep them out there long enough.