Maximum number of citations per sentence? 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.