Office365 Domain Limit

 Typically, Up to 900 domains can be added to your Office 365 subscription.

 Under the following circumstances, the limit might be lower:

• Skype for Business Online has a limit of 600 SIP domains.
  
• The following scenario is unlikely, but good to keep in mind if you have a large number of Office 365 domains. Verified domains are stored as tenant-specific information, and there is currently a limitation as to how much tenant-specific information Office 365 can store for a given tenant. Using various Office 365 features can contribute to this limit and prevent you from adding 900 verified domains to your tenant.

 for more info visit MS KB