5.1 is the (maybe) most usual HT set-up, 6.1, add one surrond at the back, 7.1, add two back surrounds, plus the normal surroud speakers at right and left in the back of the audition place.
But, a nice thing about 7.1, is the possibility to use the back surround as a Stereo set-up in another room, is also possible in some receivers, to have another source in the second zone (at least my Denon). This way, you can use 5.1 in main zone, and stereo in zone 2.