If you are talking about the limitation of adding custom domain in Azure AD then the limit is as below,
- You can add no more than 5,000 managed domain names.
- If you set up all of your domains for federation with on-premises Active Directory, you can add no more than 2,500 domain names in each tenant.
You can also refer to below article.
If you are talking about adding local custom domain names in Azure Active Directory domain services, then there is no option to add any custom domain to Azure Active Directory domain services managed domain. I have confirmed this with our PG team as well.
Let me know if you have any further questions on this.
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