The embedded VR appliance is capable of handling 200 VM replications. If you wish to manage the replications just through the add-on servers, you can do that as well.
If the environment is feasible to provide resources for multiple add-on servers, you can set that up.
Remember, when replicating VMs between 2 vCenter servers target VR server is used to manage these replications. If replications are just one way, you can deploy 2 add-on servers just at the target and if replication is bidirectional, then you can configure add-on servers at both sites.
If you are replicating VMs within the same vCenter, you can register add-on servers within the same vCenter.
I hope that helps.