I work on one at a time.

I tend to make more backgroundless images to post on my site for people to download and use in their graphic groups etc.  When I'm doing a run of those I work with one model and change hair, outfit, poses and do several of those. Then move on to the next.

However, if I'm feeling creative and working on a composition of something, then I work on it until it's done and then move onto something else.  My window of creativity is so small these days that if I put something aside to work on "later", it will never get finished. 

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi