You can look up limits in the Maximum Values list, which nowadays is interactive:
For example, a VM0 has a max. number of interfaces of 4K, and a max. no. of IPsec tunnels of 2000.
As there are no HW accelerators in a FGT-VM (though, look up "vSPU"), your real limit will be set way lower by the no. of CPU cores, and type of, of your hypervisor. It totally depends on your VM infrastructure. I'd guess you could run a couple of dozens on average HW but don't take my word for it. Test, test, test.
"Kernel panic: Aiee, killing interrupt handler!"