Best way for a proper sizing in different environments

Unfortunately, that's a very tricky question to answer. The best people to address this question would be the Professional Services department, as they deal with these types of questions daily.
The best I can do is suggest you review some of the documentation, if you do not have a PSO contact that you could reach out to. We have many documents within the support portal (Support.software.dell.com) which can help. For example.

MAGE Measurements
documents.software.dell.com/.../appendix-a-measurement-of-migration-agent-for-exchange-performance

Migrating Large Public Folders
documents.software.dell.com/.../sample-migration-scenario

SQL Server Sizing Considerations
documents.software.dell.com/.../migration-manager-for-exchange-database-server

I hope this helps
Luke

Since the product is highly scalable to meet customers needs there really isn't any single document that says to use "x" amount of agents per "y" amount of mailboxes.

As Luke states below, you can review the provided documents for some direction, but contacting your PSO Contact is the best course to take for providing the answer for your environment.

"Unfortunately, that's a very tricky question to answer. The best people to address this question would be the Professional Services department, as they deal with these types of questions daily.
The best I can do is suggest you review some of the documentation, if you do not have a PSO contact that you could reach out to. We have many documents within the support portal (Support.software.dell.com) which can help. For example."

MAGE Measurements
documents.software.dell.com/.../appendix-a-measurement-of-migration-agent-for-exchange-performance

Migrating Large Public Folders
documents.software.dell.com/.../sample-migration-scenario

SQL Server Sizing Considerations
documents.software.dell.com/.../migration-manager-for-exchange-database-server