Absolute max rating on the 555 is 16 volts and it's rated at 200ma's. So figure white or blue LED's using a 12 volt supply you could get 3 in series, with 10 strings in parallel, or about 30 LED's per 555 timer (556 is just two 555s on the same chip) that's pretty much at the limit of the 555's output. That would just blink them all on or off though, if you want to have some real fun pick yourself up some shift registers to go with the 555s. Then you can do light chasing and patterns. For just a simple flashing snowman though wouldn't be too bad. If any of this is a bit over your head or you want more details lemmi know I'll flesh it out a bit for ya.