Invoke AWS CloudFormation Stack

I don't think CF offers anything natively to handle dynamic creation of alarms for multiple instances. With the assumption that the EC2 instance scale up and down:

I would suggest using Event Bridge to notify when instances are created and trigger a Lambda function which would create the required cloudwatch alarms based on the event. The same process can be reversed when instances are scaling down. You can use SQS / SNS between Event Bridge and Lambda if you wish

--Syd