It's way less about device count, and more about AP density - especially in RF challenging environments.

I pretty much just deploy WiFi as a "line of sight" technology these days in a major city. Wherever you use the wifi you need to be able to visually see the AP. Run them in low power mode so they become effectively single-room access points.

Obviously for IoT 2.4ghz stuff sitting in closets or whatever it's still fine, but with 6ghz becoming standard the "AP in every room" model is becoming more and more relevant.

wouldn't that imply you need to run ethernet to every room? couldn't you just plug everything in?

You have 120 wifi-connected devices at home?? What kind of devices? 100 smart light bulbs or something like that?

I'm just curious – I'm a relatively techy person and I have maybe 15 devices on my whole home network.

A smart home will definitely run those numbers up. I have about 60 WiFi devices and another 45 Zigbee devices and I'm only about halfway done with the house.