I have 23 connected devices, 4 wired and next to the router, and the rest wireless. Using 5 GHz on devices that support it, and 2.4 GHz for those that don't.
What's funny is that when you call as ISP to discuss pricing, they always ask how many devices you're using, but that's misleading. Most of these devices use little data when compared with Roku devices that may be streaming internet sourced video. My Nest Thermostat, Amazon Echo Dot and many other devices don't use nearly as much. And tho I have 3 tablets, they don't all see simultaneous use. ISPs need to be smarter when helping non technical users assess their needs and not base it strictly on the number of connected devices.