As per experience there is no thumb rule, its all depends on the requirement client to client, site to site and business to business.
best i have seen or achieved was 15-25 clients per AP on before Cat 9K coming to market, Cat 9K still new to market, i have some to test but at this situation we hard to get down time tme to role out and test it.
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Lot of different things to take into account:
Client data rates/ signal strength, SNR, Co Channel, interference etc
the thing you need to take into account is the uplinks as well, how big of pipe you have from AP to services clients accessing. You might only get 10mb for a single client if thats all the WAN pipe is and its the only client.
As a rule of thumb you need to design for the clients connecting to the AP and the SNR/ RSSI they require. I would work on 20-30 clients for high throughput devices connecting until more clients are AX and then its anyones guess how well that is going to work.