I work on only one at a time, partly due to having no dedicated sewing room/space, but I'm always working on others in my head at the same time. They become "next" projects, and because I'm eager to get to them, I work faster on the one I'm actually doing. Also, if I had several going, I know myself well enough that I would get bored with one or more and then concentrate only on the one I had more interest in.