5 or fiveRefer to curriculum topic: 6.3.2Legacy inter-VLAN routing requires that each VLAN be assigned to a different physical router interface. So, if there are 5 VLANs to be configured, it is necessary to have 5 physical router interfaces, one for each VLAN.​ Explanation: If using legacy inter-VLAN routing, then 5 physical router interfaces would have to be configured to support 5 VLANs. This is because each VLAN would need its own physical router interface in order to communicate with other VLANs.