The setting "Automatically" is selected by default if the firewall object is set for Gaia as the OS. If you want to set it manually, the settings depends on your traffic and resources. The setting affect the VPN kernel. Lets assume you have 1000+ remote users, which are connect to office network thorugh the mobile access blade, it creates multiple connections ( from user to gateway, from gateway to internal rdp server). So on the example environment you should increase the maximum concurrent connections, by default maximum is 25000, increase the limit 27000 (2 times 1000). You can read for more information >
Capacity Optimization and Connections Table
If you set the maximum conn manually and for somehow it reached the limit , you can run the following command to check if there is dropped connection because of connection limit.
fw ctl zdebug + drop
You may see something like that >
By the way you can can run following command to gather statistics about the connections .
fw ctl pstat
17907 total, 2214 TCP, 10855 UDP, 21 ICMP,
4817 other, 0 anticipated, 8 recovered, 8 concurrent,
68 peak concurrent
total - Since last machine boot time.
other - Other protocols (Not TCP/UDP/ICMP)
concurrent - At the time the output was taken.
peak concurrent - Since last machine boot time.