Guys, guys... give the guy a break, will ya? stop spilling opinion and inuendo as fact. Mono amps don't necessarily have more power than 2 channel, and 2 channel amps don't necessarily have more power than 4 channel.
first, the basics...
A channel is simply an output from the amplifier. For a basic stereo application, you will need 2 channels of amplified output, one for the left and one for the right.
Typically in basic car audio application without subwoofers, you will need 4 channels of amplified output, one each for the front left, front right, rear left, and rear right. How you get 4 channels of amplified output is simply a question of taste, and there is no "right" answer. You can use the 4 amplified outputs from an in-dash source (radio, CD, tape, DVD, etc) or you can use external amplification for just two (front left/right or rear left/right) or all four corners of the car.
If you want external power for all four corners of the car, you can use two channels of amplification (one channel for the left, one channel for the right) but you will lose the ability to adjust the volume between the front and rear speakers since they will be powered from the same source. To maintain the ability to "fade" the sound from front to rear as well as control the "balance" from left to right, you will need 4 channels of amplification.
These 4 channels of amplification can come from four 1-channel amps (also called "mono" amps), or from two 2-channel amps, or from one 4-channel amp. Again, there is no "right" answer. The determining factors should be budget, space available for the equipment, desired complexity/simplicity of the install, power requirements (controlled by listening habits, listening material, and taste; has nothing to do, yet, with 12v power requirements of the amplifier(s) to deliver the performance it is/they are capable of.).
If a subwoofer or multiple subwoofers are desired to fill out the bottom octave or two of sound that typical car speakers are not capable of realistically reproducing, additional channels of amplification will be required. the solutions get a little more confusing, variable and complex at this point for the basic system, but suffice it to say that again there is no "right" answer. It is also time to consult a car audio professional to discuss the specifics of your particular application and desires. Also remember that when talking subwoofers, it is impossible to get a good solution without considering the speaker and its enclosure as a complete package. An awesome subwoofer in the wrong enclosure will sound horrible whereas an average subwoofer in its proper enclosure can sound quite good. But what is the proper enclosure? It depends on 2 things: the speaker's specs and the type of music you listen to.
But I digress.... that is getting in to things other than what you asked about....
Let me know if you have any additional questions....
