Let the editor decide.
Send your story.
Editors can't buy a story you don't send.
They know more than you do about what they want.
And you are almost certainly not a great judge of your own stories.
They may like a story that you think is not your best.
Many writers and editors are horrified by this advice.
They are concerned,
I think,
that writers will overwhelm editors with crap.
If your story is nowhere near the "quality" that an editor needs
(whatever "quality" means),
they will know within a paragraph or two.
And they likely have slush readers for that.
Err in favor of sending the story.
Let the editor decide.
Know the market.
If you read each journal,
you know something about the kinds of stories they buy.
Read each journal's submission guidelines.
Those will describe (among other things)
what genres they are interested in.
They are often quite good at expressing
what kinds of stories they don't want.
Unless you have a strong reason
to believe that a particular journal
does not want this kind of story,
lean toward sending it.
Err in favor of sending the story.
Let the editor decide.
How many to send.
As for how many to send out:
Read each journal's subscription guidelines.
There are two phrases to look for.
Multiple submissions
means submitting more than one story to the same journal.
Some journals are happy to have more than one of your stories
in their queue.
Others want you to send them only one at a time.
Still others ask that you wait for a short time
after a rejection
before you send them another story.
Simultaneous submissions
means submitting your story to more than one journal at a time.
Many journals clearly forbid simultaneous submissions
in their guidelines.
(And some writers ignore this.)
Some journals explicitly say that simultaneous okay.
They usually request
that you inform them immediately if you sell your story elsewhere.
Some writers ignore the no simultaneous submissions rule.
But when editors make clear what they want,
I try to abide by that.
If a journal accepts multiple submissions,
send them as many stories as you can write.