I'm working on a literature review for a masters thesis in the sciences, and I have a ton of sources that cover the topic. Many specific points are covered ad nauseam, and many similar ideas can be cited multiple times. My question is how many is acceptable? Is there a general rule for citing sources per sentence/idea? Does more look better? Basically, is there a limit?
The short and somewhat unsatisfactory answer is: enough. There is no formal limit but obviously too many becomes impractical. If you can reference a huge number of references for a single statement (sentence) it is normal to pick one or possibly a few by using a format indicating these references are just examples:
(e.g., Smith et al., 1943; Turner and Anthony, 1963)
some statement by, for example, Smith et al. (1943) and Turner and Anthony (1963)
I am assuming Harvard style formatting in these examples.
Exactly when it is reasonable to show examples and when one actually have to show al references is a matter of context. If you, for example, have a series of references that together build up some matter and where none is more important than the other and none summarize the other, it could be necessary to list them all regardless of how many there are. I suggest you try to look at a number of different papers of a similar type (literature review) to the one you are writing to see how others handle such instances. You should also look at other masters thesis if you have the possibility.
The main point of this is to know when it is sufficient to list only (good) examples rather than all possible references. This is of course a matter of training and learning to assess when which format is appropriate. It is therefore necessary to assess when papers simply duplicate each other (from whatever view point you reference) or when they each contribute something unique that merits their reference.
And to add to Peter Jansson, don't over do it. A literature review in an article is meant as a general reference, so the reader can get "up to speed" in the state of the art of the topic under discussion. In your thesis, you have to show that you are able to search the literature, you understand it, and are able to extract the important information.
If you put every single article, you are not fulfilling any. On the one hand, the reader will not know what are the most relevant articles for your work. On the other front, anyone can get all the articles published in a subfield in the last couple of years and write a sentence, based on the abstract and the figures, in just a few days.
In short, show that you have comprehended the literature by finding the most informative subset of articles.
Besides whether or not the citations themselves are necessary, it's also important to consider how they affect the flow of your writing.
If it is actually necessary to cite all those papers (see Is there such thing as too many references for one paper?, as well as the other excellent answers to this question), it would look much cleaner to place the citations in a footnote (especially if you use the author-year rather than numbered style).
If you use numbered citations and LaTeX, the
sort&compress option in
natbib can reduce clutter by citing a range (ie [7-16]) of sources rather than [7,8,9 ... ]. However its effectiveness depends on how your bibliography is ordered.
If you must cite many papers in a single sentence, you should at least make every effort to ensure it reads fluently and that the citations do not distract you from the main text.
Some more concrete thoughts (background is biological sciences):
A "particular point" may be covered ad nauseum in backgrounds and introductions of papers in the field, but where did that idea originate?
Cite review papers sparingly. I generally cite reviews only when pointing out that there's a tangentially related body of work well covered in someone else's review.
The only time to cite MANY sources for a particular point is when the point is (or is considered) controversial.