Each led takes 3 byte, these 3 byte contain the brightness of the 3 colors.
They are in one serial. The ws2812 takes the first 3 byte and sends the other data to the other leds
So if you want to control 4 Leds you have to send 4*3 byte, the first bytes go to the first led and the other 9 bytes go to the next led who also takes the first 3 bytes, and sends the other 6 byte to the next en so on.
If you don't understand my explanation maybe the datasheet I posted would help. On the last page it's explained how the data reaches each LED.
But to control all the leds you always have to send all the data through because each led takes 24 bit (3 byte) and sends the next through.
The data off which led that needs to light up and which color is saved in the EEPROM