Dynamis_dk:
- Is there a maximum number of LEDs any given Arduino can support?
Yes. In total the processor can handle 200 mA. Each pin has an absolute maximum rating of 40 mA but a good rule of thumb is to keep the current below 20 mA. There are per-port current limits as well. Obviously, you are also limited by the number of pins.
If the 12 LEDs can be all lit simultaneously, using the per-pin limit of 20 mA, the processor current would be 12 * 20 = 240 which is too much. This would very likely also exceed per-port limits. If you keep the per-pin current below 10 mA, 12 * 10 = 120 mA you should be fine.
- When doing a little research I came across a comment suggesting that you can bridge something on the arduino unit to disable to unit from resetting on serial connection? Is this true?
Yes. 10 uF or higher capacitor for the Uno. 120 ohm resistor for the Duemilanove. Or, cut the RESET-EN trace with a knife.
Would there be any real advantage in this?
Yes. When the processor resets all the data is cleared to startup values. By disabling RESET you can have a program with state.
realistically what kind of time does the unit take to reinitialise to accept another instruction via serial?
Approximately one second.