We need to set Property through az functionapp plan update
We are using Premium Plan.
The links
https://docs.microsoft.com/en-us/azure/app-service/manage-scale-per-app
does not talk about these 2 property accurate names...
per-site-scaling? per_site_scaling? perSiteScaling?
per-site-scaling is a feature in app service not for the azure functions app.
Maximum function app instances In addition to the plan maximum instance count, you can configure a per-app maximum. The app maximum can be configured using the app scale limit.
In-order to increase the Maximum scale out limit of app scale out under a function app use the below command
az resource update --resource-type Microsoft.Web/sites -g <RESOURCE_GROUP> -n <FUNCTION_APP-NAME>/config/web --set properties.functionAppScaleLimit=<SCALE_LIMIT>
Here is the more information about the maximum scale out limit for app scale out in function app