Is there any Linux command/Bash script for wireless clients to know the number of clients connected to each WiFi SSID ? How can I do this in bash ?
I am not sure what you want to achieve, so I will give you several answers to point in different directions.
To get all SSIDs (including all public information) you can use the following ...
$ su
# iwlist scan
I hope this is what you looking for.
If not, then you maybe want to know how many active connections one service is carring. This is by far more complicated. To check for active connections you will need a port sniffler like tcpdump and huge knowlegde.
A more common way would be to access the log files or the protocol of the server, but I think you had not asked if you have the rights to do so.