Since the days of mono, it's been known that to produce a perceived change that the majority of listeners would recognize takes an approximate doubling of speakers. Mono to stereo, stereo to Quad/5.1, and so forth, with the practical maximum for the typical home theater landing around 10-12, relating to room size and seat count. However, the specific channel/speaker plan also makes a significant difference. While research had clearly shown that localization is best in the frontal plane, and somewhat extended to the entire frontal hemisphere, indicating that more speakers would be best deployed in that hemisphere where the would do the most good, the brilliant marketing wizards have chosen to add speakers where localization is worst...behind and above. That's a rather sad error! Those developing multichannel film sound recognized very early (as early as 1938) that phantom imaging doesn't work in multi-seat rooms for anyone but those seated directly on the centerline, and those seats at home are few to none. Hence, the most used and important speaker in a theater system, home or commercial, the Center. The boffins at Dolby knew this of course, but instead they completely ignored that problem in the rear when we moved past the short-lived 6.1 plan into the ill-supported 7.1 layout (for the home, not the theater which was different).
Even worse, Atmos (especially the home version with a limited number of height speakers) depends on phantom imaging between a speaker pair for the placement of objects directly overhead, again ignoring the fact that phantom center imaging doesn't work. It's strange! So 5.1.2 or 7.2.4 tends to mis-localize height objects depending on where one is seated, and places even more speakers where they are not as easy to localize.
What I'd actually like to have is more along the lines of the 10.2/12.2 plan suggested in 1999 by Tom Holman and Chris Kyriakakis: LCR, plus Lwide and Rwide (expanding and localizing the front soundstage), then L High, R High complete the front, thus placing channels where they can actually be localized well. Surround in that plan is Ls, Rs, and Surround Back, a single rear surround speaker, taking care of the center-rear phantom image issue, or in the 12.2 plan adds 2 more surrounds and differentiates between diffuse surround and direct surround. I've heard that system several times, and it works WAY better than any home Atmos installation in rendering an impression of an acoustic space. It is challenged if there is a desire to mix a track where an object takes a zig-zag overhead path from the back to the front, of course (that could have been fixed). Unfortunately, 10.2/12.2 was never adopted, so now we're stuck with what we have.
Then we have the practical matter. The average 5.1 system I see in friends homes has at least 1, typically several, misplaced speakers that are where they are for practical or aesthetic reasons. And very few have gone to 7.x. I haven't checked market penetration for Atmos home installations, but I strongly doubt it's more than a percent or so. It's just difficult to do, and expensive not only to get the gear but to wire it. Then there's the little problem that not all films provide a lot of height stimulus. So people do what the sales guys do with in-store demos--crank up the heights so they can be HEARD!
As a result, I typically strongly suggest 5.1, with 2 or more subs (I'm sorry, I still can't get myself to call that 5.2!) because it's doable, affordable, and can be done well. A well done 5.1 system actually will be more pleasing with a far higher WAF than slap-dash systems with higher channel count, once you surmount the more-is-better bias.
At the high end we have Trinnov who has technology that mitigates the placement problem for heights and surrounds by applying sophisticated psychoacoustic processing to virtually position surround and height speakers that can't be put where they should optimally be. But that is a somewhat rarified world, as the processor along costs like a reasonable car, or a nice car, depending on how you configure it.
So I'm at "good 5.1", with dipole/tripole surrounds if possible as a first recommendation. And it gets very granular from there up.