In order to configure multiple Content Management (CM) instances you will have to update Sitecore configuration settings for each CM instance. Due to the way how Azure App Service is scaling itself out configuration updates are not possible and therefore Sitecore does not support scaling out your CM role in Azure PaaS platform.

Below is little more detail about Azure PaaS vs IaaS regarding this topic:

Azure PaaS

When you scale out to multiple instances Azure App Service simply reuses existing code and configuration to create a new instance. As you are unable to perform required configuration update, any attempt to scale out to multiple instances will create a mess with services that are communicating with the Content Management role. You can have multiple App Services as that will allow you to update Sitecore configuration file on them but each can only be scaled out to 1 instance. Remember, multiple App Services with 1 instance each = YES, single App Service with multiple instances = NO.

Azure IaaS

Generally speaking, this is the same as OnPrem Sitecore setup. Each VM will only have a single instance so if you want to have multiple instances you can just add as many instances/VMs as you need as long as you apply necessary configuration updates as documented in the official Sitecore documentation.

You can read more about how to configure multiple Content Management instances here.

You can read more about scaling up and out Azure App Service here.

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