Not very sure about your use case, type of application and how it is expected to response to failures but using an Elastic Load Balancer with one instance may be useful. It can protect your application which is running on that instance from any unwanted situations.
You use and test an auto-scaling group with minimum and maximum value as 1 instance with an Elastic Load Balancer, auto-scaling will launch a new replacement instance and make it appear behind the load balancer and user traffic will flow to the new instance. This will happen automatically. With one instance there be some degradation of service while the new is coming up.
Later, based on your requirement if you need to add more instances to load balance the traffic to your application, you can increase your min/max values in your autoscaling group without needing to change anything.
Hope this helps.
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