Do I really need a LED, could I just use a single reflective silver sticker??
No you don't have to use LEDs, you can use anything, as long as the tracking points are brighter than the background light so it can be filtered. Reportedly Freetrack works with reflective tape being illuminated by a lightbulb, if you don't mind a light shining in your face :P, alternatively you can use invisible IR LEDs to illuminate the tape but you will need to remove the IR filter from your webcam.