You do not want to have 11 LEDs to give 24v without a resistor in series, the current draw would be very susceptible to slight changes in the diodes forward drop. I would want about a 4-5V drop across a resistor which would allow 19V/2.2 = 8.6 or 9 LEDs maximum in series. The resistor would then be (24 - 9*2.2) / 20mA = 210Ω.you can parallel a serious of LEDs to help drop the voltage: 11 LEDs with Vfwd=2.2v will get you about 24v; or 5 LEDs and then a resistor of (24-5*2.2)/20ma=470ohm.
Use a >0.5W resistor.
I'd recommend a 1k2 resistor.
True.Most people always seem to go for the maximum current (20 to 30 mA) of the LED.
IMO it reduces the reliability