APX 320 max concurrent user

Not sure that you can get a specific answer. It would depend on the band (5Ghz, 2.4Ghz), the age of the user equipment, etc. Slower, older equipment can slow everything down if you allow it on your network and it's a heavy bandwidth user. (But a thermostat, for example, which uses ancient chips and slows everything down might only broadcast once an hour so essentially have no impact.) That's not even addressing speed, latency, and overall performance. So you might technically get 60 laptops on your AP but then have 60 users all complaining that the connection is so slow as to be unusable during peak hours.

And your environment will affect it: more obstacles means slower speeds, which means fewer potential clients. Which also depends on band. (For example, I run dual 5GHz to get both upper and lower band, so don't have 2.4GHz at all.)

So my guess is they need to keep it generic because they have no way of knowing your use case. Saying "128 per radio" is probably a theoretical thing in terms of an internal table.

Yes yes, i know that already (and that is what I've reply to the client). I know that the right answer is just: "it depends". But it would be nice to have at least a number to identify the theoretical max, like "128 per radio" but supported by a proper documentation.

Btw, Thank You a lot for your answer!

I hear you. On the one hand you can't say, "Just buy four of them to be safe". But if you say "128 per radio and two radios" they will anchor on that and figure you only need one because they only have 60 employees in the building. Personally, I'd slant towards distance, materials, penetration, use-cases, etc, rather than a theoretical maximum that's probably driven by only using 8 bits for a client-id.

Even in a home installation, it's more driven by distances and materials (and does the household have four people who game and stream at the same time) than by theoretical maxima.