Atmos is an expanded sound setup.
There are three ways you can have an atmos setup right now.
1. The easiest and cheapest way is upward firing atmos speaker modules. These are speakers that are meant to fire sound upward into your ceiling which then reflects it downward. Some examples are the Elac A4's, KEF R50, Def Tech A60.
http://avproductreviews.com/elac-a4-debut-review
You simply place them on top of your front left/right and rear left/right speakers (or you can put them on their own stands) so that they reflect and then hit right in front/behind your listening area.
2. Height Speakers. These are speakers you place on the wall in front and then behind you.
IE:
https://www.svsound.com/pages/prime-elevation
3. The best, but most expensive, are in-ceiling speakers placed in the ceiling.
Atmos is all about creating a "bubble" of sound so you hear things above and around you. Instead of sound being a circle around you (IE front/side/rear) you open up the heights and hear things overhead.
When it comes to sets you can start with just two atmos speakers (either in front or back) then go to four (2 front 2 rear). If you are rich you can even go even up to like 32 speakers overhead IIRC, but for most normal people, 2 or 4 atmos speakers are fine.
You use atmos you need a receiver that supports it, as well as a receiver that will support how many speakers you plan to use.
For a 7.x.4 setup (7 = 7 normal speakers, x = subwoofer, and 4 = atmos speakers) means you need a receiver capable of powering 11 speakers. Some receivers have ways to go further with a pre amp out (so you can use one to make it go from 9 speakers to say 11)