Depending on your price range, I would say forget about Atmos and anything else beyond 7.1; especially sub $500. Aside from a few outliers, the less expensive receivers will not drive all of those speakers without noticeable distortion. (This is especially true in the Dolby and other theater modes where volume is typically less than stereo given the same db level on your receiver.) Prior to HDMI, I used a Yamaha htr-5730 (was about $250 12-15 years ago) and sounded great in my 16 X 11 bedroom. Years later, I used a Pioneer vsx-522 (about $250 in 2011) to drive the same speakers in a 3.1 setup in a carpeted basement; also sounded great.
If you want Atmos or more than a 5.1 setup, make sure that each channel can handle the speakers you are using at all frequency ranges, not just the 1khz that manufacturers love to use in marketing. Also, look for THD less than 0.1% for the entire frequency range. Features aside (bluetooth, streaming, Apple whatever, other nonsense) price matters with getting a receiver (which is both the brains and the muscle of the system).
