The number of concurrent clients per SSID per AP can be changed in the bandwith section of the SSID.
How to configure the concurrent clients per SSID
STEP 1
From Cloud Management select your "
Splash Page" through Network >>> Wifi SSID
STEP 2
Access to the Bandwidth tab and set Max concurrent users field with your desired value.
Click on Apply button to take effective your settings.

The default value is 255 users per SSID per Access point.
You can reduce or increase this limit from 0 to unlimited.
NOTE:
This limitation is applied on one SSID per AP basis.
This means that if there are 5 APs with the SSID "Guest" and the limit is set to 10 concurrent clients, at the end there might be actually 50 clients connected (max 10 per AP).