From http://community.office365.com/en-us/wikis/office_365_service_updates/974.aspx#jul2013

New Office 365 tenants are limited to synchronizing 50,000 users from an on-premises Active Directory to the cloud. If a customer has more than 50,000 users and uses the DirSync tool to synchronize those to Office 365, they would previously need to call Office 365 Technical Support to get an increase in the quota. Without the quota increase customers will see errors from the DirSync tool when they exceed the quota. We recently released an improvement to the directory quota increase mechanism. This will increase a customer’s directory quota to 300,000 objects automatically when they verify their first custom Internet domain. Customers that have more than 300,000 objects, or that do not verify any custom domains, will still need to call Office 365 Technical Support to increase their quotas. Existing customers who previously verified a custom domain may still see a quota limit of 50,000. Those customers should call Office 365 Technical Support to request a quota increase. We expect that as a result of this change, most customers will not need to call Technical Support for quota increases. For more information please read the details of the directory synchronization tool and the user object quotas on TechNet.