At this point, I know that if I connect one led (5mm), I need to put one resistor (330 Ohm). Now I want to experiement, but I have some doubts.
1-If I put 3 LED's, I need to put 3 resistors? 2-How do I know the resistor that needs to go with a determated led? 3-How many LED's can I light, with the energy that comes from Pin 12 of Arduino Uno?