The information that you have provided is not sufficient for us to provide any meaning suggestions on how to set up BE in your environment. You could have one media server or multiple media servers reporting to CAS server. It all depends on how complex your environment is. It would be better for you to contact either Symantec or a reseller and get them to have a physical inspection of your environment. Then they would be in a better position to advice you.
However, regardless of how many media servers you plan to have, it is recommended that you use physical machine(s) for them and not VM. You would have problems accessing physical devices like tape drives from a VM with BE.