how to monitor how many users access to the virtual server

See this in GUI for statistics on connections/bits/requests:

Statistics  ››  Module Statistics : Local Traffic  ››  Virtual Servers

If you are using 11.x code version, you can leverage AVR/Analytics module:

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip_analytics_implementations_11_0_0/1.html

Using this command [tmsh show sys conn ss-client-addr] we can able to monitor how many user were into Virtual severs.

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