What you have to do is look at what your device is capable of driving -- and my values are based, generically, on LEDs being driven right out of a microcontroller, and tend to be somewhat conservative.
Looking at the ATMega168 data sheet it says that a pin can sink or source 40 mA, with a total on all pins of 200 mA (this is the limit through the Vcc and GND pins). So, when calculating your resistors you need to take both values into account when using a direct connection. If you're going to run the LEDs through another device (e.g., ULN2803) then you more latitude with the number of LEDs and the brightness you can achieve.