not 100% sure about the limits for Networker 8.1 EBR VMware Backup solution.
1. How many EBR appliance are supported per vCenter
2. How many EBR appliances are supported per Networker server
1. I'm not sure if there is a maximum for the amount of EBR appliances in the vCenter, but there is a limit of 100 concurrent backup sessions in the vCenter.
2. 20 per NetWorker server
The NetWorker-VMware Integration Guide page 22 has more details on the different limts of scale.
EBR is just treated as proxy virtual machine in the virtual environment, so there is no the limitation, you could deploy as many EBR appliance in one vcenter.