I'm the kind of writer that works best when I have more than one iron in the fire. Sometimes I have a lot of irons in the fire.
Just a glance at my In Progress widget over there on the right margin and you'll see a portion of what I'm working on. And those are just the ones that have a significant portion written. (There is a folder on my laptop entitled The Trunk and another one called Bits of Tripe for things I've either set aside for the time being or started before realizing they stunk.)
But there is a point at which I start to lose my momentum and the Today I... list sits quiet for weeks at a time.
Beginning writers always want to know "How many projects should I be working on at one time?" As with everything else, there are countless answers. A portion of authors work on one project at a time. Others juggle three or four or a dozen at once.
The true answer, the only one that remains true for everyone, is this: Work on however many projects you want, as long as you continue to make progress on at least one.
For me the number of stories I can write simultaneously varies from week to week and month to month. Sometimes even day to day. And that's okay.
It's okay if one day I can only work on the sci-fi western and have to put the steampunk day-in-the-life and the alternate high fantasy aside for a while.
It's also okay if I work on all three side by side for a week or so.
As long as new words are being put down on paper (or laptop) every day I can work on however many projects I want.
But when the words start to die off, it's time to cut back and refocus.
Because finishing one story is better than working on, but never finishing, a dozen.