Here is how to see number of connected to the gateway users. Nothing special/interesting and I am sure somewhere in the SecureKnowledgeBase it is to be found but with recent licensing improvements people ask a lot about that.
VALS - real-time number of currently connected users.
PEAK - largest number of users seen since last reboot

fw tab -t userc_users -s

HOST                  NAME                               ID #VALS #PEAK #SLINKS   

localhost             userc_users                        73     1     3       0

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