Hi @IniobongNkanga-8038 ,
Thank you for posting in this community, For your questions:
1.You cannot add more than five onmicrosoft.com domains in Microsoft 365, as this is the maximum limit set by the platform. Once you reach this limit, you won't be able to create additional onmicrosoft.com domains.
2.For adding a subdomain and setting it as your root SharePoint/OneDrive URL, you will need to manage your DNS settings at your registrar's website.
Use Microsoft PowerShell to rename your domain
3.However, changing the root SharePoint URL requires using PowerShell, as the option to do this directly through the admin center is not available.
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