You will often wind up pursuing new angles and wandering off into avenues you had not first considered. If in the process of organizing your paper you do not find yourself discarding half of what you have discovered, you probably have not done enough research. You will find that, if you have taken notes on index cards and labeled them by category as you were doing the research, these cards can form the basis of your outline . The point is that the organization should be by topic, not by source . Most topics should include material from more than one source.
Most sources have material on more than one topic.
I wrote as an undergraduate ranged from 8 or 10 to 35 pages. Just do what you need to do. The answer also depends upon whether you are writing one of several essays required in a course, a single research paper, or a senior project. In each instance your instructor may provide you with more specific guidelines. It is important that you understand that the research paper is not expected to be original work on your part.