raschemmel is right about using the 5V for the LEDs, but a better solution is to see if you can power them from the 5V, 2 at a time.
To be sure, you should test the particular LEDs you have. Put two of them and a resistor (say 150 ohms or so) in series. Connect them to 5V and GND, so they light. adjust the resistor to make the LEDs as bright as you want them to be, then measure or calculate the current. You want to know the current so you don't draw too much from the 5V supply when you power all your LEDs.
The reason for putting two in parallel is that two LEDs (or 50 LEDs), take the same amount of current for the same brightness as does 1 LED. The only thing you have to do is to (a) supply sufficient voltage to satisfy the forward voltage required to light an LED, and (b) to choose the resistor to arrive at that current.
So, let's say the current required to light one LED is 20 mA. 18 of them, all in parallel will take 360 mA. If they are connected as I said above, they will take 180 mA for the same current.