First, what exactly is a "channel" and are we dealing with "Stereo" or "Surround". Most likely, your HU does not do Dolby ProLogic II(x) on the fly, although some do (See the new Calrion HUs), so I'm going to assume you're working in Stereo (2 channel) mode. Basically, stereo is two independent channels that each one recieves information, be it a duplication of the original channel, or seperate information all together (this becomes more complicated in digital/surround modes)

In short, most two channel amps are designed to power two speakers, or through monobridging you can still power both speakers. Basically, you could do just your door speakers with a two channel, or you could do door+rear speakers. A four channel amp would be best to power all the cars speakers, plus subs.