Whatever your instructor requires for the assignment. Many assignments have specified numbers of sources ("2 books, 3 scholarly journal articles, no web sites.")
Whatever will answer your question. You may need:
background sources (encyclopedias, magazine articles, web sites),
in-depth coverage (books, some articles),
current research (scholarly articles)
current events (news sources)
or other types of resources (images, video, maps...)
Concrete suggestions
Make a list of what you think you will need. You will often choose your database to select a certain kind of source, so it is useful to think about this before trying to search for information.
If you aren't sure, start with background research in encyclopedias.
