When I first started quilting, I had one project going at a time and I worked on it constantly (even through power outages, plugged the machine into a car battery, lol) until I finished. It was too confusing to have more than one. Now I'm finding I'll get a few things going and it helps in a few ways:
When I have only a certain amount of time, I can choose to work on a project that fits my timeframe;
I don't get so crazy to finish a project that I rush through, thus I'm slowing down and taking my time which results in better work;
If I get frustrated with a project I can set it aside and do something else for a bit.
I still keep it to one main project with a few smaller things going on the side.