Which host are you using?
On Rackspace, you'd take an image of your web server and store it. Setup the load balancer and configure it. Then setup AutoScale configured to use the server image and load balancer you setup.
Your stack (nginx) has nothing to do with this problem other then configuring your app (if needed) for scaling. ie: if you were using PHP as your language; you'd need to store your php session variables somewhere centrally so all the servers could access them.
It'll be different on all hosts, but any host with both load balancers and scaling, it should be perfectly doable on.