No experts are answering this question so I will have a go. At the back of your stereo head unit (the radio) you will find a wiring harness that includes power leads and wires for 4 speakers. The manual for the unit will identify the wire colour codes. There will be two wires for front left speaker, two for front right, etc. Additional speakers can be spliced in to these wires but there are some issues with power.
The wiring harness setup is explained very well here
http://www.youtube.com/watch?v=QYcriAl4CTMThe same company have a web site and the power issues for multiple speakers are discussed at
http://www.rocktheboatmarine.com/install.html under "power considerations and wiring methods".
How many speakers?
http://www.youtube.com/watch?v=aZDqzfolN0Q suggests putting in an additional amplifier for extra speakers.