This table tells you the number of people you must survey to accurately represent the views of the population under study. Accurate here means relable at the .005 reliability level. In other words, the chances of the results being incorrect is 5 in 100.
From Zemke, Ron and Kramlinger, Thomas, Figuring Things Out: A Trainer's Guide to Needs and Task Analysis, Addison-Wesley, Publishing Company, Reading, MA.