If you are planning a piece of experimental work, one of the first tasks is to figure out if the project is feasible. To do this you need an idea of the numbers of samples you need for the study. The cost of research is often proportional to the numbers you need to study, and the time it takes to accomplish the research can also depend on the numbers involved, especially if you have to wait for cases.
How do you work this out? You can get an idea from your reading of similar types of research, but in the end you need to work it out exactly as this is how you will justify you project to your supervisor, to funding bodies and to an ethics committee. What you need to show is how many samples will be needed to have a reasonable expectation that the study will give a useful result. Too few samples and the whole project is a waste of time, money and may cause harm for no benefit. Too many samples is not usually the problem, but is also equally costly in terms of money, time and ethics.
How many samples you need depends on how big a difference you are looking for and how much natural variation you can expect in the populations you are studying. This article by Martin Bland explains different methods of establishing the sample sizes for studies.
Bland M (2009) The tyranny of power: is there a better way to calculate sample size? British Medical Journal 339:b3985 http://www.bmj.com/content/339/bmj.b3985