We've discussed this before, but Ernest mentioned he's involved with 30 different stories at the same time (in different stages of development), so I thought I'd raise the question again.
How many stories can you work on at the same time before each story begins to degrade?
In my case, I've noticed that, if I edit stories for too long, my creative writing takes a nose dive and the edit revisions begin to sound wooden, so I'll always work on one story while editing or revising another, just to keep my writing fresh.
Like Ernest, I've got about six full stories lined up, but which I haven't actually begun. (I've outlined them and know where they're going, but I haven't actually committed to writing them yet.) Those are all 'in the hopper' awaiting time I can dedicate to them.
So for me, the answer is only 2 (one I'm actively writing, and one I'm revising/editing), though I'll often post others, making changes as I go, but I don't count those since there isn't a much work involved.