At least for the 2200-23000 series of appliances, there is not really a hard limit on the total number of VPN tunnels beyond the amount of available RAM in the system. In the Optimization section of the firewall object, by default there can be 200 current IKE negotiations (this increased by default to 1000 in R80.10) and 10,000 concurrent VPN tunnels. Both these values can be increased if needed at the expense of more memory utilization. So for the 5200 with 8GB of RAM I'd say it can meet your requirements for number of tunnels, although if there are numerous memory-hungry blades enabled in addition to VPN there may be a shortage of memory. If this is the case upgrading to 16GB of RAM will help.
Embedded Gaia is in its own world to some degree so I can't comment on the 600-1700 series of appliances.
Beyond just the raw number of tunnels though is how much VPN throughput the firewall can handle, in the past this generally tended to be contained by limited Internet bandwidth but this is becoming less prevalent. The 5200 does not have AES-NI hardware offload (the 5600 does however, which will increase AES throughput 4-10X), but AES should still be utilized instead of 3DES for overall efficiency.
Also running R80.10 gateway is strongly recommended for that potential amount of VPN traffic, due to the new multicore IPSec VPN capability which is enabled by default in R80.10 (sk118097). In R77.30 and earlier (except for a special R77.20 hotfix) all IPSec VPN traffic processing could only take place on one firewall worker core, which is a critical bottleneck if the VPN traffic cannot be accelerated by SecureXL.
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