So, I know Ohm's Law and that Arduino UNO R3 pins (digital 2 to 13) have a rated maximum of 40 mA, although it's better to consider that they have a maximum of 20 mA for safety reasons. If I put 2 LEDs (say red ones which have the lowest voltage of 1.8 V) with a 330 Ohm resistance to each one in parallel, meaning that (from Ohm's law) each LED needs 9,96 mA current.
mA = [ (supply voltage - LED voltage) / R] * 1000 --> mA = [ (5 V - 1,8 V) / 330 Ohm] * 1000 = 9,96 mA
That means that both LEDs draw 19,92 mA current from an individual pin (correct me if I made any mistakes with calculations or prallel and series connections). So, can I plug a total of 26 LEDs (2 to each individual pin of the 13 in total) without damaging my Arduino??? Otherwise, how can I plug in as many LEDs as possible?