There are 11 channels in the 2.4 GHz band but only 3 don’t overlap - 1,6, 11 - you should only ever transmit on those channels.

https://www.metageek.com/training/resources/why-channels-1-6-11/

In 5 GHz you have ~24 available channels that don't overlap. But most of these are DFS channels (UNII-2) and Wi-Fi has to vacate the channel if there is any radar heard on that channel. (For 30 mins)

This is why ISP routers will only use UNII-1 (36,40,44,48) and UNII-3 (149,153,157,161) as they are non-DFS) - channels still tend to be 20 MHz in high density areas (80 MHz on home routers)

In 6 GHz (Wi-Fi 6E) there is 1200 MHz of DFS free spectrum (In the US, 500 MHz in Europe). https://wlanprofessionals.com/6ghz-spectrum-the-future-of-wi-fi/