Clients share slices of time (“airtime”) on the channel(s), not whole channels or sub-channels themselves. This sharing process deals with what is called contention, where multiple associated devices are vying for time through various mechanisms (prioritized but random timers) that attempt to avoid collisions of more than one device transmitting at once.
There is often a practical limitation to the number of devices that can share an APs radio (total throughput, bitrates, the total airtime, etc) and that depends on application needs is very difficult to say for sure, but easier to rough-guess. That is usually 20-30 devices per AP radio. Some vendors have configured limits so they stop accepting more associations past a number (Aruba is 64 by default) so it doesn’t lead to further issues. Often the hard limitation is the chip the AP is using for the radios - it used to be 128 associations was the max per radio, now with newer chips and IoT (needing less data per) they have the limits raised and the chipsets can handle up to 256 (some can do more? Haven’t seen it in action).