I used to have a one-project-at-a-time rule, but I found myself itching to do something else and it ruined my enjoyment of what I was working on. I use a ping pong table for my work surface, and so I started putting a project at each end of the table and working on what I wanted. Lately, though, I have a project on either side of each end (got that?) and so I'm working on FOUR projects at a time. I am a happy camper!
Honestly though, depending on the project, I rotate around to each of them--a block at a time on each--and the roof hasn't caved in yet. Really, it's your hobby. You can do it how you really want to do it. Who cares if you have UFO's sitting around?