Version 4

    I prepared this because I couldn't find OID that fetch amount of clients per ssid.

    it's sql based custom poller .there are two files, one of them is get cpu is used just to trigger the poller and has no meaning and  the second is transform poller that contains sql query that looks in wireless.clients.details.

    you need to replace the <SSID-NAME>with the SSID you are looking for.

    Both pollers must be assigned to the device, It will sum the users from all assigned controllers to single value, so if you want per controller value you must edit the query