The correct answer is 900 domains.

Microsoft 365 tenants can add up to 900 domains that are verified for use with email addresses and identity services. This limit is set by Microsoft and applies to the tenant as a whole so administrators must remove an existing verified domain if they need to add more after reaching the limit.

1200 domains is incorrect because the supported maximum is lower than that and Microsoft documents state the limit is 900 rather than 1200.

150 domains is incorrect because that number is far below the actual supported maximum and would understate the number of domains you can register.

600 domains is incorrect because it is also below the documented maximum and does not reflect the current limit of 900 domains.

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