Hi Ahmed,
The below lines are from EXOS concepts guide 22.5, it seems only a single collector is allowed at this time .
"Configuring a Collector
To export flow records using the IPFIX protocol, you must first configure a collector. Only a single
collector is allowed. You can specify the source IP address and VR to use when sending from the switch
to a given collector. When not specified, the system defaults to the switch IP address the traffic exits."