An Azure App Service Environment is a premium tier offering for high scale applications that can isolate your application on a custom virtual network and give you more control over the compute resources allocated to your applications. To increase the scale and control over the Applications running in your ASE, you can add additional App Service Plans to your ASE. When you first work with ASE's however, it doesn't seem obvious how to add additional service plans to your ASE. When creating an additional App Service Plan, you have to have enough instances in the ASE worker pool before you are allowed to create the ASP. As you can see below, I can't select my ASE from the location list:
When you want to add an additional ASP to your ASE, you select the ASE itself as the location for the creation of the ASP. Without enough instances in your ASE worker pool, the ASE is not available in the Location drop down when creating a new ASP. First you need to increase the instance count in your ASE to provide additional instances for your ASP to manage.
Increase the Instance count of your Front End or Worker Pools. This scale operation will take several hours to complete. Once the scale operation is completed, you can now create additional App Service plans in your ASE.
Hope this helps you, It drove me crazy trying to figure this out the first few times I used ASE's