You may want to check the Cloudwatch metrics and logs to monitor the resource utilization on the machine.
You can also create an auto-scaling group and put an Application Load Balancer in front of the auto-scaling group, so that more instances will be spun up if the load on your website increases.
You may want to go through this document thoroughly - https://docs.aws.amazon.com/whitepapers/latest/best-practices-wordpress/welcome.html
In particular this sub-document - https://docs.aws.amazon.com/whitepapers/latest/best-practices-wordpress/scaling-the-web-tier.html
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