It's better to have a powerful amp turned down rather than a weak amp turned up. If you run the amp close to its max power output it won't sound good. But there is much more to it than that.
I only use 3 speakers in my car and they are all running through amps. 2x 6 inch in the front and a sub in the rear. But they are all mounted on a solid base, using good amps and wire and also running through a crossover.
It pretty much comes down to how good do you want it and at what volume. Mine is more designed around quality rather than volume. Don't get me wrong it goes loud, but it will never be "the loudest car in the world" because that isn't what's important to me.