I am creating a new pipeline in AWS and my source is CodeCommit. But when I click on create pipeline, I get an error:

Failed on the following Amazon CloudWatch Events operation: PutTargets. The following message contains details on the exception: The requested resource exceeds the maximum number allowed.

I know my event rule has 5/5 targets and there is a limit so the new target cannot be added. However, I am unable to find a way to add/link new event rule associated with CodePipeline.

According to

If you select only DEV branch in your CodeCommit source, it might add 3 targets in your event rule i.e. for DEV, DEV1 and DEV2. I don't exactly know the reason behind this, could be a bug or perhaps intentional. My bet is on regex as the culprit here. So in your case, it could be that you have more than 5 branches that start with the name DEV and that's why it is reaching the limit, hence giving you the error.

So here's what you can try. Select the branch that is either totally unique or has some differences. In other words, select any branch other than DEV and see if you still get the error. If this works, it means that it's time to properly organize your repository and have proper naming convention for the branches.