This is purly your decision, you can use single logic app for your production, test and dev scenarios. However, you need to consider the following.
Currently you may see the email sending logic is simple, but as your application grow, you may see the requirement to add additional functionality such as log the requests, or additional steps etc, in this case it is a better idea to have dev, test and production logic apps.
Based on the best practices in Software Development, make sure that you have separate environments available for development, testing and production. This will help you to do troubleshooting, add additional functionalities etc.
Hope this helps