I would personally recommend scripting this and recording the value at whatever interval you deem necessary.
On the GUI you can get this information under Network -> GlobalProtect -> Gateways and selecting the 'Remote Users' link under INFO on each gateway.
On the CLI you can simply run 'show global-protect-gateway statistics' to see quickly the user information for each gateway you have configured, importantly this will also show you to total current users that the GUI won't. This is all users connected to any of the gateways on the device.
This of course means you can also use the API call type=op&cmd=<show><global-protect-gateway><statistics></statistics></global-protect-gateway></show> to retrieve the same information programmatically and you would just record the ['response']['result']['TotalCurrentUsers'].
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