How many references do I need?

There are certain tricky questions I've developed stock answers for.
Questions like, do you write assignments, what's the point of post-modernism and how do I get crime statistics on left-handed burglars called Geoff in the UK?

But the big one is always 'how many references do I need?' The problem with the answer is the problem with the question. How do you quantify enough of anything? Enough to pass? Enough to get an 'A'? The question also presupposes that all references are equally good; they're not. So an assignment with 20 poor references probably won't be marked as highly as an assignment with 12 good quality ones. Put simply, the quantity is secondary, the quality is primary.

For dissertations this 'how many is enough' question is particularly difficult. For undergrads you're partly constrained by your word limit. You couldn't physically fit 300 references into a 6000 word dissertation even if you wanted to.
The best advice I can offer is this; look at how academics write journal articles, look at how they use referencing to construct and support their arguments and look at how often they do this. That should give you a pretty good idea of how often to reference whilst your writing, and in so doing you'll naturally end up with enough academic back up to write a decent piece of work.