Summarizing the answer from our comments to benefit the community.
The App Service Domain quota for Visual Studio Enterprise- MPN subscription is only 1 and cannot be increased.
At this time, if you wish you may share your feedback on quota increase for the' subscription on our Uservoice - All of the feedback, you share in these forums will be monitored and reviewed by the Microsoft engineering teams responsible for building Azure. Additionally, users with a similar request can up-vote you post and add their comments.
“Not all subscriptions at a given level are eligible to receive support incidents. Subscriptions acquired through FTE, NFR, MPN, MCT and other programs do not receive support incidents. If your subscription is not eligible to receive technical support incidents, the Technical Support tile will not appear in your available benefits.” -Resource
Thanks ChuckTsai-1369 for your patience and great collaboration.