May be this is a little bit advanced, but needs mentioning from time to time.
TheArduino/AVR has exactly ONE AD-Converter. That it uses 6 pins (or 8 with the SMD versions) is a kind of fake, called multiplexing. You can easily extend the number of analog inputs through a 50 cents chip called 74HC4051 to as many lines as you like if you are not too demanding in your sampling frequency...