This circuit limits the current through the LEDs to 20mA. The sensing resistor is both setting the current limit and biasing the base of the second transistor wich in turn switches the first transistor. The supply voltage is droped across the first transistor with just enought going though the LEDs to drive it at full potential. You could connect LEDs up to Vcc-Rs voltage. Rs is set at 0.707V so to drive the LEDs you have 12V-0.707V=11.29V for LEDs.
If you want to parallell LEDs you have to use high current transistors and will be able to use lower voltage.
