The 555 is capable of sinking and sourcing up to 200mA, but it gets very hot when
doing this on a 12v supply.
The following circuit shows the maximum number of white LEDs that can be
realistically driven from a 555 and we have limited the total current to about 130mA as
each LED is designed to pass about 17mA to 22mA maximum. A white LED drops a
characteristic 3.2v to 3.6v and this means only 3 LEDs can be placed in series.