I usually work on 2 projects at once. I find it gives me a mental break when I am just tired or frustrated with fmq'ing, piecing, etc. Then when I go back to the other project, clear mind and energy! I find that I lose focus when I have more than 2 projects. Too many fabrics, threads, clutter etc. for me! Do what makes you happy. It's your joy and you are the only one who can define what that is.