Whether you are investing for retirement or strictly to make money you need to be diversified. So if you are going to own individual stocks you need to make sure you have your bases covered in industry sectors.
Basic industries
Energy
Public utilities
Capital goods
Consumer durables
Consumer non-durables
Consumer services
Finance
Health care
Technology
Transportation
To be minimally diversified with stocks you need to have at least 5 stocks and they must represent 5 different industry sectors. By doing this you will not be hurt as much if one sector becomes out of favor.