Hi,
Thank you for posting in Microsoft Q&A forum.
1,To migrate the primary server to a new infrastructure in order to update the OS and SQL, we can use site Backup and Restore method and use the FQDN and IP address of the new server are the same as the old one. Helpful articles for your reference:
ConfigMgr Site Backup and Restore
MIGRATE YOUR SCCM SERVER TO A NEWER SERVER
ConfigMgr SCCM Server Hardware Migration Step by Step Guide to Perform
Administrator checklists for migration planning in Configuration Manager
2,You can refer to the official article to determine the site size and scale:
Size and scale numbers for Configuration Manager
Thanks for your time. Have a nice day!
Best regards,
Simon
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