In ATX specifcations any rail can give out a maximum of 240W. So a 12v line's maximum allowed amperage would be 20A. 12V*20A= 240W. If that amount is exceeded the power supply's protections should kick in and shut down the power supply.
Power supplies work in the way that some internal components are shared among the rails. Basically this means that any rail will not give out it's maximum output unless it is the only operating rail. Those reported amps are a theoretical maximum.
Not usually. There are a few PSU's that you can do that with. On most PSU labels there's a second line that will tell you the total power you can pull of the 12 volt rail.
One way to get around that is to use a single rail PSU such as Corsair or PC P&C. The single rail technically violates the ATX specification but Corsair (and I, having two Corsair PSU's) do not worry about it.