It’s expected to have a lot of L2 extended (stretched) port groups to be in use by enterprise application workloads. You can use HCX, preferred, or standalone NSX Edge to extend required networks to VMware Cloud on AWS.
Take care while designing the traffic flow for VMs residing on an L2 extended port-group: the default gateway of your stretched network will still be located on-premises. Network traffic send to other routed or extended port groups configured in VMware Cloud on AWS will first reach the default gateway of your extended network, located on-premises, and then will be routed back to VMware Cloud on AWS. It might cause additional latency and load on your WAN link. Introduction of the proximity routing feature to VMware Cloud on AWS (in limited preview now) will help to overcome this situation.
Scale the VMware HCX Extension Service (if used for L2 extension) for high-density environments; the HCX-NET- EXT/L2E component can be deployed for each extended network, providing about 4–6 Gbps per extended network. Check the HCX User Guide for more details.
It's not recommended to stretch non-routable, specific purpose-built networks (like cluster heartbeats) if all nodes using such network are migrated to VMware Cloud on AWS. Rather create a new network in VMware Cloud on AWS and re-assign port-groups during migration.