How many different projects should one handle or juggle at any given time frame? The answer - there is no right answer, only the right answer for you.
For me i really enjoy doing sketches and concepts. Doing these helps me learn new techniques and it helps to get me warmed up and the creative juices flowing. The pieces i hold onto and like i will try to finish, but i try to focus on only 2-3 pieces at one time. Doing it this way helps to step away from a project when i get stuck or don't know the exact direction i'm going. This process also gives me time to step away to get a fresh eye. Working on a single project sometimes you don't step back and take different views of the piece as a whole, which can reveal certain problem areas.