Chapter 15 Unified Threat Management for FortiOS 5.0 : Intrusion protection : Configure IPS options : Configuring the IPS engine-count
  
Configuring the IPS engine-count
FortiGate units with multiple processors can run more than one IPS engine concurrently. The engine-count CLI command allows you to specify how many IPS engines are used at the same time:
config ips global
set engine-count <int>
end
The recommended and default setting is 0, which allows the FortiGate unit to determine the optimum number of IPS engines.