Hi Dave,
Depends how you have scaling configured. For example, with Manual you set the number of instances and the count stays constant, regardless of activity.
With Rules-based (aka Custom autoscale) when it scales out/in is based on the rules you create. For example, you could set it so that if CPU percentage stays over 80% for 5 minutes to add an additional instance. CPU percentage is just one example, you can choose from many different metrics.
With Automatic scaling it scales out/in based on incoming HTTP requests. This is relatively new scaling option that requires that you use Premium SKU.
Automatic scaling in Azure App Service
https://learn.microsoft.com/en-us/azure/app-service/manage-automatic-scaling
-TP