How many channels do I actually need in an AV receiver?

For a basic 5.1 system, a 5.2-channel receiver is enough. If you want Dolby Atmos with height speakers, look for 7.2 channels (5.1.2 layout) at minimum, and 9.2 or 11.2 if you want a full 7.1.4 or 5.1.4 Atmos setup. Reviewers in r/hometheater repeatedly note that paying up for more channels is mainly worthwhile if you actually plan to install ceiling or height speakers.