Scale Azure Container Instance
It is not possible to scale a specific ACI instance. If you wanted more CPU/Memory you would need to redeploy that container.
You can Horizontally scale ACI, i.e. add more containers, however, ACI does not support load balancers, so you would need to manage the collection of IP addresses you would be provided with. Of course, you could deploy a frontend container to spread the load.
As mentioned in comments, you can connect AKS to ACI, and use Kubernetes to handle orchestration and scale. Unfortunately, I don't believe Windows is supported under AKS currently (though that should be arriving very soon I imagine)
Depending on your use case, you may be better deploying to a standalone cluster while AKS / ACI catch up with what you need.
You can use an ARM template to easily deploy multiple Azure container instances. Have a look at the 'copy' function in particular.
Then you can use Azure DNS traffic manager to load balance between the various containers.