SSL Certificates for B2B setup

We are in process of ordering SSL certificate for B2B setup. We have two Reverse Gateway servers in DMZ load balanced using External Load balancer ( Virtual ).
They are connected to two Internal Integration Servers in Cluster. Its a standard B2B setup recommended by WebMethods.

Our external load balancer has IP/Server name which is used by external partners for communication.

How should we order cerificates ? Should we order 1 license for external load balancer which is getting verified by CA or we should order 4 licenses since certificate deployed on 4 servers i.e. 2 RG servers and 2 Internal Servers ?

Thanks,
Prashant

Single set of certificates can be used on Gateway and Internal servers.