Well...What I am not getting is...how does it matter that how many IP addresses you have used for IP spoofing? That has nothing to do with your load test in a load balanced environment or in that case regardless of how many App servers you may have. All you will be doing is hitting the VIP for the Load Balancer. The AppServer will only see these VIPs. Again...Correct me if I am wrong.
The way Windows NLBS works is that it load balances connections to a selected preferential node if the requests are all coming from a single IP address. If you are going to perform your load testing, you would have to somehow genereate load from different IP addresses to evenly distribute the load.
I'd say the more the merrier, can't think of a reason why not. Depending on the rules of the load balancer, if you've only got 2 ip addresses, they may both end up going to the same server. The more IP addresses you use, the more chance you give the load balancer of doing it's job.
Some load balancers (Cisco for example) can apply a mask (e.g.255.255.255.240)to the IP address to ensure that all requests that come from a group of IP addresses are sent to the same server. This is because people like AOL may switch your IP address between requests.
If you can, try and make your range of IP addresses as large as possible.
One other thing to look out for it you network path. if you multiplex from your agents through an internal network to an external network and then back in you will have to pick up a different IP address upon exiting the internal network (this can be an issue when you are sending the load from a different location than where the load balancer sits). I do this quite often.