A 5.1 system consists of five main channels. You connect three speakers at the front, one in the middle and the other two on the left and right. In addition, you'll need two speakers that you place at the height of your sofa on the left and right sides for ambient sound.
For even more encompassing sound, go with the 7.1 system. Since the seven speakers are arranged in a circle around you as an audience member, you'll get sound from all sides, even from behind.