I've heard 1 reference per 100 words is a decent rule of thumb to go by. However, I personally take the opinion that the number of references you should have is the number of references you need to clearly explain your subject area and give the reader and appropriate breadth and depth of understanding. Adding references for the sake of adhering to the rule of thumb I just mentioned does nobody any favours. If it doesn't add anything to your work then it's unlikely to be worth doing. However you may wish to also include a bibliography of additional sources.
My 10,000 word masters thesis had somewhere between 10 and 20 references and about 10 items in the bibliography and this was appropriate given the nature of my work. Other people I knew had nearly 200 references in theirs.