When adding a custom domain, the error “You've reached the maximum usage limit for the single certificate domain” appears.

This error occurs when you attempt to add a new domain after the maximum number of registered domains has been reached.
Custom domains using Salesforce CDN have a default limit of 10 per organization.
Please try the following solutions in order.

Solution 1: Delete Unnecessary Custom Domains

You can free up space to add new domains by checking your currently registered domains and deleting any unused ones.
Navigate to [Setup] | [User Interface] | [Sites and Domains] | [Domains] and delete any unnecessary domains.

Solution 2: Request a limit increase from Salesforce Support

If you have no unnecessary domains and all are required, you can request an increase in the limit from support.
Create an inquiry and include the following information along with the error message you received.
Please apply for the exact number of extensions you need.

1. Organization ID (Create case per organization)
2. Is this a request from a system administrator representing the organization? (Yes/No)
3. Applicant's username (email address format)
4. Desired maximum number after expansion