Answers to your Questions

If I want to simulate hundreds of VPN customers, do I have to test with hundreds of test ports?

No, a single test port should be able to emulate many customer sites. For example, a single port could emulate thousands of Ethernet VLANs or ATM PVCs. If the tester supports routing protocol sessions over these virtual interfaces, each one could emulate a connection to a customer edge router in a BGP/MPLS VPN implementation. You could then measure how increasing the number of these virtual interfaces affects the router's forwarding performance.