Taking a whack at it using my overly rusty knowledge.
If you set things up like audioguru described, 3 LEDs in a chain with a 150ohm resistor and assuming the LEDs have a 3V drop across each of them.
You end up having 3V across your 150 ohm resistor, current equals voltage over resistance (I=V/R), so your current draw per 3-LED chain is 3/150 A
Or, 20mA, meaning if you max out the suppy you should be able to run 50 chains (150 LEDs) but as was said, it's not recommended that you max things out.
Of course, you also said they're 20mA LEDs, so you'd set up the circuit to put close to 20mA through each LED, which means you'll always have 50 chains whether you go with a 1, 2 or 3 LED chain (changing the size of the inline resistor with each chain size)