In general, if you are using ELBs you should have at least 2 instances per AZ or cross zone load balancing enabled. I've seen this get teams several times.
The other thing to consider when deploying to the cloud with load balancers is to use an immutable architecture. Taking hosts out of service, updating them, and putting them back in service is a bit cumbersome at best and leaves you vulnerable to service outages.
The other thing to consider when deploying to the cloud with load balancers is to use an immutable architecture. Taking hosts out of service, updating them, and putting them back in service is a bit cumbersome at best and leaves you vulnerable to service outages.