power dissipated by a device is equal to the voltage across that device multiplied by the current passing through it.
First, you're going to need a 487 Ohm resistor to get 20mA for a 2.25V diode drop.
The voltage across the resistor will be 9.75V. You have a 0.5W resistor. I always suggest a margin of safety of 2 in terms of power dissipation of resistors, so consider it to be 0.25W max power dissipation from the resistor. That would mean the max current through the resistor is 0.25W/9.75V = 25.6mA. The max number of resistors you can run per LED is 1. If you want to increase that to 2 resistors per LED, you need to up the power rating to at least 1W. (.04^2*487 = 0.78W)