Hello @Jacek Slazak !
Hope you are having a great day!
Thank you for asking a Question! We are Glad to Assist you!.
how many existing domains can I add ?
You can add up to 900 domains to your subscription (including subdomains). You can't add a domain to Microsoft 365 that you're already using in another Microsoft cloud service. That means that you can't add the same domain to multiple subscriptions.
For more information you can visit the link mentioned below
https://learn.microsoft.com/en-us/office365/servicedescriptions/office-365-platform-service-description/domains
Let me know if the above solution resolved your issue!
Regards,
Tasadduq Burney
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