When you put more batteries in parallel, you increase the amount of current (amps) you can draw; this can put off the time where the battery voltage drops, but not forever.
The problem with using a 12-volt battery on a device that runs on 12 volts is that over time, the battery voltage will drop below 12 volts, as Audioguru pointed out. At some point, the voltage will be too low for the device (camera, whatever) to function properly. How low can you go? We don't really know; the only way to really find out is to try it. It probably won't damage the camera, but at some point, it will start exhibiting flaky behavior, or will stop working altogether.
Ideally, you would use a voltage source (battery) that's higher than the voltage you need by some significant amount (say, 14-15 volts), then regulate the voltage down to 12 volts. This will give you a longer run time before the battery voltage "sags" to an unusably low level.
Does that make sense?