At least 5. If you want this values just as a reference, 5 concentrations should do fine. More points will be better for finding sensitivity differences between groups with slight changes. At the end it will depend on your objectives and how much you know about the dose-response effect of the substance. I normally use seven points when carrying out initial tests. Once you have a good idea of the dose response behaviour of the drug in the cell line in question, you may use less points and still get a relatively robust result.