You are supposed to be limited to the license that you bought, for instance, an 8U box can handle 8 calls. A 48 U box, 48 calls. If you plan on having the calls go to AA, be aware of the platform limitations in calls to AA and VM. You have 6 sessions there on a UC520.
By bandwidth alone you are looking for about 80k per g711 call and 24k per g729 call. There are other factors including what else you are doing on the box. If you have a lot of VPN traffic, this could be less as it will take away some CPU cycles.
Another factor is how good is your QoS on the link? Is the link 10Mbps up and down? What else is going on the link? QoS can be setup outbound on the router, but you need to make sure you also have inbound QoS through your ITSP as well. I hope this helps.
