Multichannel movie sound dates back to Disney's "Fantasia." When the film was first released in 1940, the number of speakers used was scaled to the size of the individual theater; 30 to 80 speakers were installed behind the screen and around the perimeter of the ceiling.
Home theater multichannel sound arrived many decades later, and quickly settled on a 5.1 channel system, which is just a scaled-down version of the current movie theater system. The home system uses three front speakers--left, center, right--and two surround speakers placed on the sides of the room. The subwoofer supplies LFE (low-frequency effects) and the low bass the satellite speakers can't reproduce on their own.
Five-channel home theater does a great job for most people, but since we Americans usually favor a more-is-better approach, a number of companies tried to add more speakers to the mix. Seven-channel receivers started showing up in the early 2000s to take advantage of the DTS ES and Dolby EX six- and seven-channel formats. Six-channel systems added a single rear speaker; seven-channel systems use two rear surround channel speakers. Since most people have their couch up against or near the rear wall, adding rear speakers is problematic. Very, very few owners of 7.1 channel receivers actually use seven speakers. That makes sense; in all but the largest home theaters, 7.1 channel systems offer little performance advantage over 5.1 systems.
Maybe that's why Dolby cooked up Pro-Logic IIz in 2009. That 7.1 (or 9.1 channel) system uses front "height" speakers that, in theory at least, produce a more lifelike surround experience. My CNET listening tests proved otherwise; the height speakers didn't make a discernable difference. Worse yet, I couldn't hear the speakers making any sound at all, so I increased the height speakers' volume again and again. No matter what I tried, I could not hear any difference.
My experiences with Audyssey DSX were somewhat more positive. DSX is a 7.1, 9.1, or 11.1 channel system that uses height and/or additional left and right front speakers to broaden the front soundstage. I still didn't think that the height speakers contributed anything, but the DSX width speakers created a more spacious sound effect.
Of course, buying better-sounding speakers, instead of adding more speakers, is the best way to improve any home theater's sound quality. What do you think is the optimum number of speakers for a home theater?