De La Soul says 3 is the magic number.
In the design world, however, the magic number is 7 plus or minus 2 (so more of a magic range, I suppose). The reason is simple: apparently people can only remember or process 7 +/- 2 pieces of information at a time (George A.Miller, 1956). So, we IAs set about ensuring our designs had only 5 to 9 options in a navigation system or 5 to 9 tabs on a screen.