For all FortiGate-6000 and 7000 models, the CLI allows you to add up to 512 flow rules. However, the number of flow rules that you can add is actually limited by the FortiGate-6000 and 7000 internal switch hardware:
All FortiGate-6000F models support up to 256 flow rules.
All FortiGate-7000E models support up to 512 flow-rules.
All FortiGate-7000F models support up to 52 flow-rules.