4 years late, but you can get this info through ACC in 7.0 (sort one of the widgets by session and look at the graph) or through CLI:
show running resource-monitor
In the output, you will see session utilization within the last X timeframe, but indicated by percentage.
<snip>
Resource utilization (%) during last 13 weeks:
session (average):
0 0 0 0 0
session (maximum):
1 1 0 0 0
</snip>
To see what your maximum session supported count is on the firewall, you can issue this command:
admin@PA-200> show session info
--------------------------------------------------------------------------------
Number of sessions supported: 65534
You can roughly figure out the session usage through CLI by taking the percentage of the maximum supported session number.
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