Box 1: 4 -
Two public IP addresses in the on-premises data center, and two public IP addresses in the VNET.
The most reliable option is to combine the active-active gateways on both your network and Azure, as shown in the diagram below.
Box 2: 2 -
Every Azure VPN gateway consists of two instances in an active-standby configuration. For any planned maintenance or unplanned disruption that happens to the active instance, the standby instance would take over (failover) automatically, and resume the S2S VPN or VNet-to-VNet connections.
Box 3: 2 -
Dual-redundancy: active-active VPN gateways for both Azure and on-premises networks
Reference:
https://docs.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-highlyavailable