This circuit is based around the 555 timer chip operating in astable mode.
The rate of flashing is set by the 2 resistors and capacitor to the left of the timer chip.
The output of the chip is connected to 2 sets of Leds via transistors. The top set uses a pnp transistor and the bottom set uses an npn transistor. This would allow for the addition of a lot more leds as the 555 timer chip can only sink and source up to 250 mA max.
In this configuration the timer chip is only sinking a few milliamps and so will never get very hot and it's lifetime will be greatly increased.
If you have any questions or suggestions for improvement please comment.
