Is there any limit to have the email domains added when you select the below option :
I am going to have 300+ domains hence want to make sure its supported.
Serving external customers from specific email domains
After setting up your help centers to serve external customers, you can further restrict them to serve only customers from selected email domains.
To do so:
Go to Settings () > Products > Jira Service Management > Customer access.
In Account types section under External, check the box that says Allow portal-only accounts to be created for new customers accessing the help center.
Check the box that says Only allow account creation for customers with specific email domains.
Add in specific email domains of your choice.