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it's actually easy to know, usually it is written on the cans themselves (in the form of a symbol for recycling I think). you can check with a magnet.
an LED is basically 20mA / 3V, and that is 60mW. a 12V 1A supply is 12W, so you _could_ power 200 of them in the best case. In the worst case (all LEDs in parallel with each their own current limiting resistor), 20mA per LED -> 50 LEDs.You want to drive parallel strips of 3/4 LEDS with just one limiting resistor. That would make it 150-200 LEDs. I would probably go with strips of 3.