If you need version-12 and version-13, you’ll need to add two benches.
Single bench is easiest to get started for servers with very limited resources. No point in using it if your server has better resources available.
In case of multiple benches as well as multiple servers use Docker Swarm or Kubernetes setup.
Even if it is a single high performance server use docker swarm for multiple benches setup.
That way you get the portainer UI and a common traefik setup for allowing multiple services over 80/443 for multiple domain names. You can host frappe benches as well as other stacks using this setup.
Multi-bench vs Multi-server
Bench is set of processes. Two benches means two instances of these processes.
In terms of Docker it means 2 docker-compose projects or 2 Docker Swarm stacks
In terms of Kubernetes it means 2 helm releases.
Instead of using 1 VM or HPC to host all benches, cluster of servers is used to host the processes.
In case of multi server setup separate the database, redis and files on separate server for better management.