There quite a few threads already going into 'How-to-Build Police flashers' here. Don't remember if any were 12 volt, although a couple were automotive. Use the search, but think most are under cleaver disguise as 'Emergency flasher', or something similar, since running these on a public road is usually consider illegal. Basically, you use a 555 timer, or gated logic for a clock signal, a 4017 counter will give you 10 outputs per chip, but you can cascade more. Only one output will be on at a time, and it advances one per clock. The first pin could turn on your LEDs, the second pin would be skipped to give you an off period. You could hook up several different patterns, colors, with this basic set up. You will need a driver section to power 50 LEDs, transistor or mosfet..
Anyway, no sense in re-inventing the wheel, your project, you search. Post a link back when you find something you need further help on. Personally, I used an AVR microcontroller for my flashing police lights (use to mess with my neighbor).