You need a channel per speaker, that includes the sub. You can run a single 5 channel amp or a 4 channel amp with a mono amp for the sub. Ok, get ready for some umbrella statements... (Disclaimer: not
always true)
Single amp solutions (5 channel):
*Easier to install
*Space efficient
*Cost effective
*Limited availability and selection
*Normally a lower ceiling for power output
Multi amp solutions (4ch + mono):
*Flexible upgrades
*More selection
*Normally a larger footprint
*Higher ceiling for power output
Different 5 channel amps have different uses for their 5th channel as well. Some are designed for a center channel speaker (ie Alpine MRA-F350) while other are designed for a sub (ie Apine PDX-5). This is determined by their power output on the 5th channel.
Keep in mind!! Many will tell you that you need huge subs with gobs of power and this is simply untrue. In reality, many setups over 500w and a single nice 10"/12" sub are there for showoff (this assumes you are using decent to nice equipment and not kingwood

).
And read read read!