It depends what power pack ( or battery) voltage you have for you project box.
I take it yo have mounted a reed switch to every door & window and run wires from each to your box. A lot of wires, did you have fun laying them?
You can calculate your own resistor value:
battery (or power pack) voltage minus 2.2 or 3.5 gives you the voltage drop at the resistor (Vr).
Resistance value is Vr divided by .02 (20 mA).
So, for a 9 V battery, its 9V-2.2V = 6.8V / .02A = 340 ohm. The nearest standard resistor value is 330 ohm, use that.
Since that resistor is dissipating power you need to know that too. Power (watts) is voltage x current, for my example its 6.8 x .02 = 0.14 watt. Get the nearest bigger standard wattage, 0.25W.
Klaus