but could you add as many of these as you wish?
Not usually because shields of the same type use the same pins and there is no addressing mechanism. It can be done but I doubt that would be the case with most (all?) Arduino shields.
Does this also go for the RS232 shields?
All shields although one of two have jumpers to get around this.
I mean, there must be some limit of the number of serial ports you could have, right?
Not really but the right hardware has to be created.
are these shields some kind of an extension of the UARTs you have on the Arduino itself,
or are they standalone serial ports?
I've not seen a shield with it's own UARTs.
do they magically appear as COM x ?
COM x has no meaning in an Arduino.
One more question; is the Arduino suitable for use in very low temperatures? By that I mean under -30C for instance.
No, and no hardware is south of a lot more $$$ will be, certainly nothing you buy from Sparkfun et al. That's way too cold.
How about you tell us more about the project?
For example why do you need so many serial ports, can't you make up a network.