based on the vSphere Replication FAQ, there is a limit of 500 protected vm's per an appliance.
can i use multiple vSRA Appliances registered against the same vCenter and by doing so, replicate 2000 VM's (as an example)
It is possible to register only one vSphere Replication Appliance per single VC. This is enforced limitation. If you try to deploy new one it will overwrite the old and will clear your active replication.
The 500 replications limit is the supported one and not hardcoded limitation.
You can have at most one vSphere Replication Management Server (VRMS) per one vCenter Server and the limit of 500 incoming+outgoing replications is per VRMS.
If using VR together with SRM, you can deploy additional VR-server-only appliances (w/o VRMS in them) for better routing of replication traffic to the target datastores (better locality of VR server to hosts that have access to the target datastores).