Current State:

Currently, Jira Service Management Cloud has a hard limit of 10 email domains that can be associated with a single customer organization for automatic grouping.

Doc: https://support.atlassian.com/jira-service-management-cloud/docs/use-email-domains-to-automatically-group-customers-into-organizations 

Proposed Future State:

• Increase the hard limit of 10 domains to a higher value (e.g., 50 or 100).

• Provide a way for administrators to request a limit increase for specific high-scale organizations.

Workaround:

There is no known workaround to this problem right now.