if you get one you are going to need a 2 channel amp. if you get 2 you only need a mono. they are dual 8 ohm subs. you can only wire them down to 4 ohms. either get a 2 channel that bridges to 300rms or buy 2 shivas and buy a mono 600rms amp.
That information is incorrect. You can run 1 or 2 subs with either 1 or 2 channel amp. You just have to match up the impedance to make sure the amp can handle it. Kicker KX600.1 would be a terrific match for that sub and if you get another one, it'll supply just as much power to the 2nd sub.
From a budget standpoint, it makes sense to buy a 2 channel amp if you only buy one Shiva. Assuming you only bought one Shiva, it's cheaper to buy a 2 channel amp that does the rated power on a bridged load than it is to buy a mono amp and run a 4 ohm load on it. The Shiva will run well on either, it's just a matter of how much you want to spend. The 2 channel amp, being AB, would offer better sound quality than a mono amp, but draw more current doing it.