This is probably one of the most frequently asked questions, but one for which there is not always a short answer.
Of course, there is a theoretical technical limit for the maximum number of concurrent associations, which is 128 per radio (so for a dual-radio model 128+128=256), but plan your network based upon that, and neither you nor your users will be very happy.
The following does not replace a site survey but gives you some guidance as to the recommended number of clients for each APX model based upon three factors:
A few examples:
If you're a customer we would advise you to work with your local Sophos partner to plan your deployment. If you're a Sophos partner and don't have the tools to do a survey, your local Sophos SE or (in some regions) your distributor can provide further help.
Considering the price of these against opposition equivalent models I would expect way better performance.