In addition to the user licenses, you usually need more environments than what are included in the licenses. Only if you use the application without any customizations is it feasible to not need more environments.
- Each developer needs one development system
- Each developer needs a Visual Studio subscription / license
- If you want to use automatic testing, you might need additional Visual Studio or Azure DevOps licenses / addons
- One build system is needed - but with a recent new feature you can use "build as a service" in Azure DevOps. But even that costs something.
- You might need additional test systems (for example if you want to have a system where you can drop a copy of the prod database at any time to troubleshoot some urgent issue)
- You might need a (usually temporary) performance test system
- You might need an integration platform such as Logic Apps to integrate between D365FO and other systems
- You might want to export data to external database (Data Lake, Azure SQL) for reporting / analysis purposes
Since it's a broad topic, perhaps it's good to discuss it with your partner since they can help you evaluate what additional components you might need.
