Wireless Access

on Aruba WLC (e.g. 7220-series), what command can show max number of wifi clients per access point (if such  default value exists per default config settings or per applied configured settings)

Valid for any AOS version, these are covered in the AOS CLI Reference Guide.

At the SSID level, one can look at the configured value:

max-clients  0-255, default is 64

(cx7010-1) [mynode] #show wlan ssid-profile <SSID name> | include Max
Max Transmit Attempts                                   8
Max Associations                                        64 <----------------------------------------- configured value
Maximum Transmit Failures                               0

One can also query an AP and see the values for all SSIDS

(cx7010-1) [mynode] #show ap config ap-name <apname> | include "Source,Max Associations"
Parameter                                                                    2.4GHz                           5GHz                             5GHz-secondary                   6GHz                             Source
Parameter                                                       2.4GHz            5GHz              5GHz-secondary    6GHz              Source
Max Associations                                                64                64                64                64                wlan ssid-profile "SSID"

Also 

show ap debug received-config ap-name <apname>