AT on my VPC CIDR for traffic traversing a VPN connection

Openswan in AWS

I am following this video below because AWS doesn't support

https://www.youtube.com/watch?v=Ov7cHlcIkHc

NAT on my VPC CIDR for traffic traversing a VPN connection? according to this article

https://aws.amazon.com/premiumsupport/knowledge-center/configure-nat-for-vpn-traffic/

The third party vendor does not accept private IPs so I am thinking of using open. I just a bit confused as the network guy said we need two public IPs. I believe on we will get attached to the opens server to establish the VP but what is the second one for ? We have a server in a AWS private subnet that I don't want to attach a public elastic IP to. Does the SWAN server need two elastic IPs?

they are expecting a public IP from us so I'm assuming the private ip will be translated from the private to the public they are expecting. and coming in who will will VPC know to or swan know to translate their public ip to the private ip server. just trying to understand how many public ips the swan server needs. how will the swan server get the data coming from their a public ip?