Regardless of how many speakers you have, if you listen in stereo (2 channel), only the front two speakers will output sound. If you use DSP's, Dolby digital, DTS decoding, or 5/7 channel stereo, you will get a combination of some or all speakers outputting sound. Many people find that 2ch stereo sounds better for music.
Ideally you want the front two speaker to form a triangle with where you sit.
So if you are 8' back, the L/R speakers would be about 8' apart, measured from the centers of the speakers. You also want to avoid being too close to the walls, in order to prevent reflections. If possible, try to avoid having your fronts within ~2 feet of the side walls.
Also, ideally, your speakers (all of them) would be setup an equal distance from your primary seating position. So if you were 8' back from the fronts, your surrounds would each be 8' from you. The surrounds for 5.1 would be placed to your side or slightly behind you, in the 90-135 degree positions and ~2' above you. With 7.1, the side surrounds would be ~ the 90deg position and the rear surrounds would be ~the 135deg position.
Most people don't have a room layout to place speakers in those ideal spots, so you have to figure out what works best for you and what you prefer the sound of. My fronts are 8' from where I sit & about 5.5' apart. My side surrounds are ~4.5' to my side and my rear surrounds are 5.5' back. Not even close to a circle, but that's about what my room allows.
Jack