With 9V, you can connect LEDs in series.
4 LEDs in series will drop approximately 6-8 Volts (depending on exactly which LEDs you're using).
Add a resistor to limit it to 20ma. (If your LEDs use 6.8 V, the remainder is 2.2V that the resistor has to drop. 2.2V/0.02A = 110 ohms.)
Make 5 strings like this for a total of 20 LEDs.
Total current draw is 5 *20ma = 100ma.