If your instructor does not provide source number guidelines in the instructions for an assignment, you can ask them how many sources they expect you to use.
A rough guideline you can also use is that you should have about as many sources as there are pages in your paper. For example, for a six page paper you should expect to use about six sources. Fewer than that, and you may not have enough to write about; more than that, and your paper may just be a summary of the sources, with no room for your own ideas.