how long does a short story have to be??

However long you want it to be. There isn't a set minimum of anything. It's up to you.

How long is a piece of string?

There is no set minimum, though obviously one line doesn't really a full story make, and over 20,000 words is closer to a novella than a short story. It's up to you to figure out what you want your writing to be.

Quality over quantity is a good rule of thumb.