Hi @Marco S. Junior ,
regarding your question #1:
You don't need an Azure support plan per subscription but per Azure billing account.
What does the monthly cost over?
The monthly support plan cost covers one billing account, regardless of how many subscriptions or users are on the account. All subscriptions under a billing account share the same support plan, and all users with admin or owner access to any of the subscriptions under the account with a support plan are entitled to support for those subscriptions. Go to the Azure portal to see the list of subscriptions in your account.
Source: Azure support plans FAQregarding your questions in #2:
There is no "right" or "wrong" how you structure your landing zone subscriptions.
One of our customers is running the landing zone using different Azure subscriptions for each environment: Landing Zones A2-DEV, Landing Zones A2-PROD, Landing Zones A2-TEST
Another customer separates PROD, DEV and TEST just by different vNets/NSG/Firwall in one Azure subscription.
Both approaches are valid in my opinion. The design should be based on the individual requirements.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten