The flow-session resource is needed in order to ensure adequate and secure communication between the two logical systems.
The flow-session resource must be defined in the security profile for the interconnect logical system because the interconnect logical system is responsible for forwarding traffic between other logical systems. The flow-session resource determines the maximum number of sessions that the interconnect logical system can create and maintain. If the flow-session resource is not allocated or is insufficient, the interconnect logical system might drop packets or fail to establish sessions1.
The NAT resources are not needed to be allocated to the interconnect logical system because the interconnect logical system does not perform any NAT operations on the traffic. The NAT resources are only relevant for the logical systems that need to translate the source or destination IP addresses or ports of the traffic1.
No resources are not needed to be allocated to the interconnect logical system is incorrect because the interconnect logical system still requires some resources to function properly, such as the flow-session resource. The interconnect logical system cannot operate without any resources allocated to it1.
The resources must be calculated based on the amount of traffic that will flow between the logical systems is partially correct, but not the best answer. The resources must be calculated based on the amount of traffic and the type of traffic that will flow between the logical systems. For example, the flow-session resource depends on the number and duration of sessions, the security-log-stream-number resource depends on the number and size of logs, and the NAT resource depends on the number and type of NAT rules1.
Reference:
Security Profiles for Logical Systems | Junos OS | Juniper Networks