@Marc Arlt TP is correct, the underlying webspaces are different so you won't be able to move them around using the default tools. In these cases, you'll need to manually deploy the web apps into a single ASP.
I would advise against this though. Trying to run 21 web apps on a single ASP could start to push the limits. Each web app requires its own overhead processes. Also, if you use deployment slots, each of those are technically a web app with their own overhead processes so you may have more than 21 web apps. Each web app is different so there is no guidance on the maximum number of web apps but when I was a support engineer 20 was a common number for an S1 tier web app to start having performance issues and requiring the customer to move a few web apps into a second ASP to avoid availability issues due to resources being stressed.
My point is that you might be taking on an expensive endeavor in terms of time only to have to undo it depending on the size of your ASP instance and your web apps resource requirements. I would ensure you do significant research to understand how both your current ASP's are behaving in terms of resources before moving forward.
I hope this adds some further insights into what TP has already shared. Please let us know if you have any further questions or concerns.
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