Elastic bean deployment with RDS

Hi,

I am considering a solution to auto scale my database Postgres with Elastic bean deployment of Flask application. I recently went through guides and tutorials that RDS provides stand by database for multi AZ for failover. However, In below guide, it is advised to have 2 databases to avoid single point failure.

"Finally, configure your environment's Auto Scaling group with a higher minimum instance count. Run at least two instances at all times to prevent the web servers in your environment from being a single point of failure, and to allow you to deploy changes without taking your site out of service."

My question is if we are enabling multi AZ with stand by instance, why to run 2 instances all the time. Can anyone help me what is the correct understanding.