The resistors will depend on what kind of LEDs and how many there are in the circuit, how they're wired and how you're powering the circuit (how much voltage). In an ideal circuit, each LED gets its own resistor, maybe 1 for every 2 if you're trying to save some pennies and space. When you have a pre-made circuit with a bunch of LEDs and you don't want to open the product to modify it internally, or you can't, then things get a little more hairy, because now you have to place a larger resistor to cut the maximum current to the entire circuit to make it "safer" - that means possibly reducing the light output. Even hairier is the fact that you don't know the specs of the LEDs and have to make guesses based solely on their color and size (this last one if you can get a clear look at them).