Increase custom domain limit

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**Tell us about your request**

There is currently a limit of 5 custom domains. My application use-case requires many more. I think a reasonable limit would be more like 500 custom domains.

**Describe alternatives you've considered**

Heroku

**Additional context**

**Attachments**

Start by reviewing AWS App Runner's current custom-domain quota and how service limits are managed; the issue names no repository files, tests, or entry points. Compare the existing limit of 5 with the requested capacity and determine the service-level work needed for a higher limit. Done means the supported custom-domain limit is increased and its behavior is documented.

Written by the indexing model from the issue text.