One way to approach this question is to say that there is no clear answer as to how much literature is enough. But we can, just as a thought experiment, try a few different answers and see what insights come out.
Let's start with a reasonable guess... Let's say you need 200 references in total throughout your thesis. The next question is which 200? In many fields, there are thousands upon thousands of possibly relevant sources. So even if you include 200, you'll still have to exclude most of them.
Now let's go to an extreme and say you need 2000 references. This lets you cover a lot of ground, but it will be impossible to cover anything in depth. If you were to give a summary of each individual source, you'd drown in detail and there would be no space left for your own original work, so what you would have to do is write about the literature as a whole. Taken collectively, what does the research say? What are the trends in the literature and what are the kinds of questions being asked?
Finally, let's go to the other extreme and say you can only include 20 and no more. What can you absolutely not leave out? Which sources are crucial to your work? Which are the best, most relevant, most useful? These you could go into in much greater detail.
Although these numbers are clearly arbitrary, they do tell us something useful about the skills you need for a good literature review.
- You need to leave out most of the literature and focus on the best and most interesting work
- You need to be able to summarise wide areas of the literature by focusing on the trends, rather than individual papers
- You need to know which sources are the most important and say more about these